UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

FORM N-CSR

CERTIFIED SHAREHOLDER REPORT OF REGISTERED MANAGEMENT

INVESTMENT COMPANIES

Investment Company Act file number            811-04873            

                           The GAMCO Growth Fund                          

(Exact name of registrant as specified in charter)

One Corporate Center

                         Rye, New York 10580-1422                            

(Address of principal executive offices) (Zip code)

Bruce N. Alpert

Gabelli Funds, LLC

One Corporate Center

                         Rye, New York 10580-1422                            

(Name and address of agent for service)

Registrant’s telephone number, including area code:  1-800-422-3554

Date of fiscal year end:   December 31

Date of reporting period:   June 30, 2017

Form N-CSR is to be used by management investment companies to file reports with the Commission not later than 10 days after the transmission to stockholders of any report that is required to be transmitted to stockholders under Rule 30e-1 under the Investment Company Act of 1940 (17 CFR 270.30e-1). The Commission may use the information provided on Form N-CSR in its regulatory, disclosure review, inspection, and policymaking roles.

A registrant is required to disclose the information specified by Form N-CSR, and the Commission will make this information public. A registrant is not required to respond to the collection of information contained in Form N-CSR unless the Form displays a currently valid Office of Management and Budget (“OMB”) control number. Please direct comments concerning the accuracy of the information collection burden estimate and any suggestions for reducing the burden to Secretary, Securities and Exchange Commission, 450 Fifth Street, NW, Washington, DC 20549-0609. The OMB has reviewed this collection of information under the clearance requirements of 44 U.S.C. § 3507.

Item 1. Reports to Stockholders.

The Report to Shareholders is attached herewith.

To Our Shareholders,

For the six months ended June 30, 2017, the net asset value (“NAV”) per Class I Share of The GAMCO Growth Fund increased 14.6% compared with increases of 9.3% and 14.0% for the Standard & Poor’s (“S&P”) 500 Index and the Russell 1000 Growth Index, respectively. See below for additional performance information.

Enclosed are the financial statements, including the schedule of investments, as of June 30, 2017.

Comparative Results

The GAMCO Growth Fund

Disclosure of Fund Expenses (Unaudited)

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Summary of Portfolio Holdings (Unaudited)

The following table presents portfolio holdings as a percent of net assets as of June 30, 2017:

The GAMCO Growth Fund

The Fund files a complete schedule of portfolio holdings with the Securities and Exchange Commission (the “SEC”) for the first and third quarters of each fiscal year on Form N-Q. Shareholders may obtain this information at www.gabelli.com or by calling the Fund at 800-GABELLI (800-422-3554).The Fund’s Form N-Q is available on the SEC’s website at www.sec.gov and may also be reviewed and copied at the SEC’s Public Reference Room in Washington, DC. Information on the operation of the Public Reference Room may be obtained by calling 800-SEC-0330.

Proxy Voting

The Fund files Form N-PX with its complete proxy voting record for the twelve months ended June 30, no later than August 31 of each year. A description of the Fund’s proxy voting policies, procedures, and how the Fund voted proxies relating to portfolio securities is available without charge, upon request, by (i) calling 800-GABELLI (800-422-3554); (ii) writing to The Gabelli Funds at One Corporate Center, Rye, NY 10580-1422; or (iii) visiting the SEC’s website at www.sec.gov.

Portfolio Manager Biography

Howard F. Ward, CFA, joined Gabelli Funds in 1995 and currently serves as GAMCO’s Chief Investment Officer of Growth Equities as well as a Gabelli Funds, LLC portfolio manager for several funds within the Gabelli/GAMCO Fund Complex. Prior to joining Gabelli, Mr. Ward served as Managing Director and Lead Portfolio Manager for several Scudder mutual funds. He also was an Investment Officer in the Institutional Investment Department with Brown Brothers, Harriman & Co. for four years. Mr. Ward received his B.A. in Economics from Northwestern University.

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The GAMCO Growth Fund

Schedule of Investments — June 30, 2017 (Unaudited)

See accompanying notes to financial statements.

4

The GAMCO Growth Fund

See accompanying notes to financial statements.

5

The GAMCO Growth Fund

Statement of Changes in Net Assets

See accompanying notes to financial statements.

6

The GAMCO Growth Fund

Financial Highlights

Selected data for a share of beneficial interest outstanding throughout each period:

See accompanying notes to financial statements.

7

The GAMCO Growth Fund

Notes to Financial Statements (Unaudited)

1. Organization. The GAMCO Growth Fund was organized on October 24, 1986 as a Massachusetts business trust and commenced investment operations on April 10, 1987. The Fund is a diversified open-end management investment company registered under the Investment Company Act of 1940, as amended (the “1940 Act”). The Fund’s primary objective is capital appreciation.

2. Significant Accounting Policies. As an investment company, the Fund follows the investment company accounting and reporting guidance, which is part of U.S. generally accepted accounting principles (“GAAP”) that may require the use of management estimates and assumptions in the preparation of its financial statements. Actual results could differ from those estimates. The following is a summary of significant accounting policies followed by the Fund in the preparation of its financial statements.

Security Valuation. Portfolio securities listed or traded on a nationally recognized securities exchange or traded in the U.S. over-the-counter market for which market quotations are readily available are valued at the last quoted sale price or a market’s official closing price as of the close of business on the day the securities are being valued. If there were no sales that day, the security is valued at the average of the closing bid and asked prices or, if there were no asked prices quoted on that day, then the security is valued at the closing bid price on that day. If no bid or asked prices are quoted on such day, the security is valued at the most recently available price or, if the Board of Trustees (the “Board”) so determines, by such other method as the Board shall determine in good faith to reflect its fair market value. Portfolio securities traded on more than one national securities exchange or market are valued according to the broadest and most representative market, as determined by Gabelli Funds, LLC (the “Adviser”).

Portfolio securities primarily traded on a foreign market are generally valued at the preceding closing values of such securities on the relevant market, but may be fair valued pursuant to procedures established by the Board if market conditions change significantly after the close of the foreign market, but prior to the close of business on the day the securities are being valued. Debt obligations for which market quotations are readily available are valued at the average of the latest bid and asked prices. If there were no asked prices quoted on such day, the security is valued using the closing bid price, unless the Board determines such amount does not reflect the securities’ fair value, in which case these securities will be fair valued as determined by the Board. Certain securities are valued principally using dealer quotations.

Securities and assets for which market quotations are not readily available are fair valued as determined by the Board. Fair valuation methodologies and procedures may include, but are not limited to: analysis and review of available financial and non-financial information about the company; comparisons with the valuation and changes in valuation of similar securities, including a comparison of foreign securities with the equivalent U.S. dollar value American Depositary Receipt securities at the close of the U.S. exchange; and evaluation of any other information that could be indicative of the value of the security.

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The GAMCO Growth Fund

Notes to Financial Statements (Unaudited) (Continued)

The inputs and valuation techniques used to measure fair value of the Fund’s investments are summarized into three levels as described in the hierarchy below:

A financial instrument’s level within the fair value hierarchy is based on the lowest level of any input both individually and in the aggregate that is significant to the fair value measurement. The inputs or methodology used for valuing securities are not necessarily an indication of the risk associated with investing in those securities. The summary of the Fund’s investments in securities by inputs used to value the Fund’s investments as of June 30, 2017 is as follows:

The Fund did not have transfers between Level 1 and Level 2 during the six months ended June 30, 2017. The Fund’s policy is to recognize transfers among Levels as of the beginning of the reporting period.

There were no Level 3 investments held at June 30, 2017 or December 31, 2016.

Additional Information to Evaluate Qualitative Information.

General. The Fund uses recognized industry pricing services – approved by the Board and unaffiliated with the Adviser – to value most of its securities, and uses broker quotes provided by market makers of securities not valued by these and other recognized pricing sources. Several different pricing feeds are received to value domestic equity securities, international equity securities, preferred equity securities, and fixed income securities. The data within these feeds are ultimately sourced from major stock exchanges and trading systems where these securities trade. The prices supplied by external sources are checked by obtaining quotations or actual transaction prices from market participants. If a price obtained from the pricing source is deemed unreliable, prices will be sought from another pricing service or from a broker/dealer that trades that security or similar securities.

Fair Valuation. Fair valued securities may be common or preferred equities, warrants, options, rights, or fixed income obligations. Where appropriate, Level 3 securities are those for which market quotations are not available, such as securities not traded for several days, or for which current bids are not available, or which are restricted as to transfer. When fair valuing a security, factors to consider include recent prices of comparable securities that are publicly traded, reliable prices of securities not publicly traded, the use of valuation models,

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The GAMCO Growth Fund

Notes to Financial Statements (Unaudited) (Continued)

current analyst reports, valuing the income or cash flow of the issuer, or cost if the preceding factors do not apply. The circumstances of Level 3 securities are frequently monitored to determine if fair valuation measures continue to apply.

The Adviser reports quarterly to the Board the results of the application of fair valuation policies and procedures. These may include back testing the prices realized in subsequent trades of these fair valued securities to fair values previously recognized.

Foreign Currency Translations. The books and records of the Fund are maintained in U.S. dollars. Foreign currencies, investments, and other assets and liabilities are translated into U.S. dollars at current exchange rates. Purchases and sales of investment securities, income, and expenses are translated at the exchange rate prevailing on the respective dates of such transactions. Unrealized gains and losses that result from changes in foreign exchange rates and/or changes in market prices of securities have been included in unrealized appreciation/depreciation on investments and foreign currency translations. Net realized foreign currency gains and losses resulting from changes in exchange rates include foreign currency gains and losses between trade date and settlement date on investment securities transactions, foreign currency transactions, and the difference between the amounts of interest and dividends recorded on the books of the Fund and the amounts actually received. The portion of foreign currency gains and losses related to fluctuation in exchange rates between the initial purchase trade date and subsequent sale trade date is included in realized gain/(loss) on investments.

Foreign Securities. The Fund may directly purchase securities of foreign issuers. Investing in securities of foreign issuers involves special risks not typically associated with investing in securities of U.S. issuers. The risks include possible revaluation of currencies, the inability to repatriate funds, less complete financial information about companies, and possible future adverse political and economic developments. Moreover, securities of many foreign issuers and their markets may be less liquid and their prices more volatile than of securities of comparable U.S. issuers.

Foreign Taxes. The Fund may be subject to foreign taxes on income, gains on investments, or currency repatriation, a portion of which may be recoverable. The Fund will accrue such taxes and recoveries as applicable, based upon its current interpretation of tax rules and regulations that exist in the markets in which it invests.

Securities Transactions and Investment Income. Securities transactions are accounted for on the trade date with realized gain/(loss) on investments determined by using the identified cost method. Interest income (including amortization of premium and accretion of discount) is recorded on the accrual basis. Premiums and discounts on debt securities are amortized using the effective yield to maturity method. Dividend income is recorded on the ex-dividend date, except for certain dividends from foreign securities that are recorded as soon after the ex-dividend date as the Fund becomes aware of such dividends.

Determination of Net Asset Value and Calculation of Expenses. Certain administrative expenses are common to, and allocated among, various affiliated funds. Such allocations are made on the basis of each fund’s average net assets or other criteria directly affecting the expenses as determined by the Adviser pursuant to procedures established by the Board.

In calculating the NAV per share of each class, investment income, realized and unrealized gains and losses, redemption fees, and expenses other than class specific expenses are allocated daily to each class of shares

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The GAMCO Growth Fund

Notes to Financial Statements (Unaudited) (Continued)

based upon the proportion of net assets of each class at the beginning of each day. Distribution expenses are borne solely by the class incurring the expense.

Distributions to Shareholders. Distributions to shareholders are recorded on the ex-dividend date. Distributions to shareholders are based on income and capital gains as determined in accordance with federal income tax regulations, which may differ from income and capital gains as determined under GAAP. These differences are primarily due to differing treatments of income and gains on various investment securities held by the Fund and timing differences. These book/tax differences are either temporary or permanent in nature. To the extent these differences are permanent, adjustments are made to the appropriate capital accounts in the period when the differences arise. These reclassifications have no impact on the NAV of the Fund.

The tax character of distributions paid during the year ended December 31, 2016 was as follows:

Provision for Income Taxes. The Fund intends to continue to qualify as a regulated investment company under Subchapter M of the Internal Revenue Code of 1986, as amended (the “Code”). It is the policy of the Fund to comply with the requirements of the Code applicable to regulated investment companies and to distribute substantially all of its net investment company taxable income and net capital gains. Therefore, no provision for federal income taxes is required.

The Fund is permitted to carry capital losses forward for an unlimited period. Capital losses that are carried forward will retain their character as either short term or long term capital losses.

The following summarizes the tax cost of investments and the related net unrealized appreciation at June 30, 2017:

The Fund is required to evaluate tax positions taken or expected to be taken in the course of preparing the Fund’s tax returns to determine whether the tax positions are “more-likely-than-not” of being sustained by the applicable tax authority. Income tax and related interest and penalties would be recognized by the Fund as tax expense in the Statement of Operations if the tax positions were deemed not to meet the more-likely-than-not threshold. For the six months ended June 30, 2017, the Fund did not incur any income tax, interest, or penalties. As of June 30, 2017, the Adviser has reviewed all open tax years and concluded that there was no impact to the Fund’s net assets or results of operations. The Fund’s federal and state tax returns for the prior three fiscal years remain open, subject to examination. On an ongoing basis, the Adviser will monitor the Fund’s tax positions to determine if adjustments to this conclusion are necessary.

3. Investment Advisory Agreement and Other Transactions. The Fund has entered into an investment advisory agreement (the “Advisory Agreement”) with the Adviser which provides that the Fund will pay the Adviser a fee, computed daily and paid monthly, at the annual rate of 1.00% of the value of its average daily net assets. In accordance with the Advisory Agreement, the Adviser provides a continuous investment program

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The GAMCO Growth Fund

Notes to Financial Statements (Unaudited) (Continued)

for the Fund’s portfolio, oversees the administration of all aspects of the Fund’s business and affairs, and pays the compensation of all Officers and Trustees of the Fund who are affiliated persons of the Adviser.

The Fund pays each Trustee who is not considered an affiliated person an annual retainer of $9,000 plus $2,000 for each Board meeting attended. Each Trustee is reimbursed by the Fund for any out of pocket expenses incurred in attending meetings. All Board committee members receive $500 per meeting attended. The Chairman of the Audit Committee and the Lead Trustee each receives an annual fee of $2,000. The Chairman of the Proxy Voting Committee and Nominating Committee each receives a $1,000 annual fee. A Trustee may receive a single meeting fee, allocated among the participating funds, for attending certain meetings held on behalf of multiple funds. Trustees who are directors or employees of the Adviser or an affiliated company receive no compensation or expense reimbursement from the Fund.

4. Distribution Plan. The Fund’s Board has adopted a distribution plan (the “Plan”) for each class of shares, except for Class I Shares, pursuant to Rule 12b-1 under the 1940 Act. Under the Class AAA, Class A, and Class C Share Plans, payments are authorized to G.distributors, LLC (the “Distributor”), an affiliate of the Adviser, at annual rates of 0.25%, 0.25%, and 1.00%, respectively, of the average daily net assets of those classes, the annual limitations under each Plan. Such payments are accrued daily and paid monthly.

5. Portfolio Securities. Purchases and sales of securities during the six months ended June 30, 2017, other than short term securities and U.S. Government obligations, aggregated $121,377,669 and $146,666,666, respectively.

6. Transactions with Affiliates and Other Arrangements. During the six months ended June 30, 2017, the Distributor retained a total of $594 from investors representing commissions (sales charges and underwriting fees) on sales and redemptions of Fund shares.

During the six months ended June 30, 2017, the Fund received credits from a designated broker who agreed to pay certain Fund operating expenses. The amount of such expenses paid through this directed brokerage arrangement during this period was $2,377.

The cost of calculating the Fund’s NAV per share is a Fund expense pursuant to the Advisory Agreement. During the six months ended June 30, 2017, the Fund paid or accrued $22,500 to the Adviser in connection with the cost of computing the Fund’s NAV.

7. Line of Credit. The Fund participates in an unsecured line of credit, which expires on March 8, 2018 and may be renewed annually, of up to $75,000,000 under which it may borrow up to 10% of net assets from the custodian for temporary borrowing purposes. Borrowings under this arrangement bear interest at a floating rate equal to the higher of the overnight Federal Funds rate plus 125 basis points or the 30 day LIBOR plus 125 basis points in effect on that day. This amount, if any, would be included in “Interest expense” in the Statement of Operations. During the six months ended June 30, 2017, there were no borrowings outstanding under the line of credit.

8. Shares of Beneficial Interest. The Fund offers four classes of shares – Class AAA Shares, Class A Shares, Class C Shares, and Class I Shares. Class AAA and Class I Shares are offered without a sales charge. Class A Shares are subject to a maximum front-end sales charge of 5.75%, and Class C Shares are subject to a 1.00% contingent deferred sales charge for one year after purchase.

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The GAMCO Growth Fund

Notes to Financial Statements (Unaudited) (Continued)

The Fund imposes a redemption fee of 2.00% on all classes of shares that are redeemed or exchanged on or before the seventh day after the date of a purchase. The redemption fee is deducted from the proceeds otherwise payable to the redeeming shareholders and is retained by the Fund as an increase in paid-in capital. The redemption fees retained by the Fund during the six months ended June 30, 2017 and the year ended December 31, 2016, if any, can be found in the Statement of Changes in Net Assets under Redemption Fees.

Transactions in shares of beneficial interest were as follows:

9. Indemnifications. The Fund enters into contracts that contain a variety of indemnifications. The Fund’s maximum exposure under these arrangements is unknown. However, the Fund has not had prior claims or losses pursuant to these contracts. Management has reviewed the Fund’s existing contracts and expects the risk of loss to be remote.

10. Subsequent Events. On July 5, 2017, the Fund began to offer for sale Class T Shares.

Management has evaluated the impact on the Fund of all other subsequent events occurring through the date the financial statements were issued and has determined that there were no other subsequent events requiring recognition or disclosure in the financial statements.

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The GAMCO Growth Fund

Board Consideration and Re-Approval of Investment Advisory Agreements (Unaudited)

At its meeting on February 23, 2017, the Board of Trustees (“Board”) of the Fund approved the continuation of the investment advisory agreement with the Adviser for the Fund on the basis of the recommendation by the trustees who are not “interested persons” of the Fund (the “Independent Board Members”). The following paragraphs summarize the material information and factors considered by the Independent Board Members as well as their conclusions relative to such factors.

Nature, Extent, and Quality of Services. The Independent Board Members considered information regarding the portfolio manager, the depth of the analyst pool available to the Adviser and the portfolio manager, the scope of administrative, shareholder, and other services supervised or provided by the Adviser, and the absence of significant service problems reported to the Board. The Independent Board Members noted the experience, length of service, and reputation of the portfolio manager.

Investment Performance. The Independent Board Members reviewed the short, medium, and long term performance of the Fund against a peer group of large cap growth funds chosen by Broadridge as being comparable. The Independent Board Members noted that the Fund’s performance was in the second quartile of the funds in its category for the one year and five year periods and in the third quartile for the three year period.

Profitability. The Independent Board Members reviewed summary data regarding the profitability of the Fund to the Adviser both with an administrative overhead charge and without such a charge. The Independent Board Members also noted that a small portion of the Fund’s portfolio transactions were executed by an affiliated broker and that another affiliated broker received distribution fees and minor amounts of sales commissions.

Economies of Scale. The Independent Board Members discussed the major elements of the Adviser’s cost structure and the relationship of those elements to potential economies of scale.

Sharing of Economies of Scale. The Independent Board Members noted that the investment advisory fee schedule for the Fund does not take into account any potential economies of scale that may develop.

Service and Cost Comparisons. The Independent Board Members compared the expense ratios of the investment advisory fee, other expenses, and total expenses of the Fund with similar expense ratios of the peer group of large-cap growth funds and noted that the advisory fee includes substantially all administrative services for the Fund as well as investment advisory services of the Adviser. The Independent Board Members noted that the Fund’s expense ratios and the Fund’s size were above average within this group. The Independent Board Members also noted that the advisory fee structure was the same as that in effect for most of the Gabelli funds. The Independent Board Members recognized that the Adviser and its affiliates did not manage other accounts with similar strategies that had fees lower than those charged for the Fund.

Conclusions. The Independent Board Members concluded that the Fund enjoyed highly experienced portfolio management services, good ancillary services, and a reasonable performance record. The Independent Board Members also concluded that the Fund’s expense ratios and the profitability to the Adviser of managing the Fund were reasonable and that economies of scale were not a significant factor in their thinking at this time. The Independent Board Members did not view the potential profitability of ancillary services as material to their decision. On the basis of the foregoing and without assigning particular weight to any single conclusion, the Independent Board Members determined to recommend continuation of the investment advisory agreement to the full Board.

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The GAMCO Growth Fund

Board Consideration and Re-Approval of Investment Advisory Agreements (Unaudited) (Continued)

Based on a consideration of all these factors in their totality, the Board Members, including all of the Independent Board Members, determined that the Fund’s advisory fee was fair and reasonable with respect to the quality of services provided and in light of the other factors described above that the Board Members deemed relevant. Accordingly, the Board Members determined to approve the continuation of the Fund’s Advisory Agreement. The Board Members based their decision on evaluations of all these factors as a whole and did not consider any one factor as all important or controlling.

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Item 2. Code of Ethics.

Not applicable.

Item 3. Audit Committee Financial Expert.

Not applicable.

Item 4. Principal Accountant Fees and Services.

Not applicable.

Item 5. Audit Committee of Listed Registrants.

Not applicable.

Item 6. Investments.

Item 7. Disclosure of Proxy Voting Policies and Procedures for Closed-End Management Investment Companies.

Not applicable.

Item 8. Portfolio Managers of Closed-End Management Investment Companies.

Not applicable.

Item 9. Purchases of Equity Securities by Closed-End Management Investment Company and Affiliated Purchasers.

Not applicable.

Item 10. Submission of Matters to a Vote of Security Holders.

There have been no material changes to the procedures by which the shareholders may recommend nominees to the registrant’s Board of Trustees, where those changes were implemented after the registrant last provided disclosure in response to the requirements of Item 407(c)(2)(iv) of Regulation S-K (17 CFR 229.407) (as required by Item 22(b)(15) of Schedule 14A (17 CFR 240.14a-101)), or this Item.

Item 11. Controls and Procedures.

Item 12. Exhibits.

SIGNATURES

Pursuant to the requirements of the Securities Exchange Act of 1934 and the Investment Company Act of 1940, the registrant has duly caused this report to be signed on its behalf by the undersigned, thereunto duly authorized.

Pursuant to the requirements of the Securities Exchange Act of 1934 and the Investment Company Act of 1940, this report has been signed below by the following persons on behalf of the registrant and in the capacities and on the dates indicated.

^* Print the name and title of each signing officer under his or her signature.