<?xml version="1.0"?>
<ownershipDocument>

    <schemaVersion>X0303</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2010-01-14</periodOfReport>

    <notSubjectToSection16>0</notSubjectToSection16>

    <issuer>
        <issuerCik>0001277475</issuerCik>
        <issuerName>GANDER MOUNTAIN CO</issuerName>
        <issuerTradingSymbol>GMTN</issuerTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001355719</rptOwnerCik>
            <rptOwnerName>Erickson Brian A</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>4567 AMERICAN BOULEVARD WEST</rptOwnerStreet1>
            <rptOwnerStreet2></rptOwnerStreet2>
            <rptOwnerCity>MINNEAPOLIS</rptOwnerCity>
            <rptOwnerState>MN</rptOwnerState>
            <rptOwnerZipCode>55437</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>0</isDirector>
            <isOfficer>0</isOfficer>
            <isTenPercentOwner>1</isTenPercentOwner>
            <isOther>0</isOther>
            <officerTitle></officerTitle>
            <otherText></otherText>
        </reportingOwnerRelationship>
    </reportingOwner>

    <nonDerivativeTable>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <transactionDate>
                <value>2010-01-14</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>J</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
                <footnoteId id="F1"/>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>2428847.011</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>5.15</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>9964703.011</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>I</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value>By Holiday Stationstores, Inc.</value>
                    <footnoteId id="F2"/>
                </natureOfOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
        <nonDerivativeHolding>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>1042174.926</value>
                    <footnoteId id="F3"/>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>I</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value>By Holiday/GMTN Family LLC</value>
                    <footnoteId id="F2"/>
                </natureOfOwnership>
            </ownershipNature>
        </nonDerivativeHolding>
    </nonDerivativeTable>

    <derivativeTable></derivativeTable>

    <footnotes>
        <footnote id="F1">On January 14, 2010, the Issuer effected a reverse stock split after which any shareholder that held less than one share will receive $5.15 for each pre-split share.  Pursuant to an agreement between Holiday and the Issuer, Holiday funded a portion of these cash payments in the amount of $12,508,701.15, and the Issuer has agreed to issue Holiday shares equal to this funding amount divided by $5.15 (2,428,847.011 shares).</footnote>
        <footnote id="F2">The reporting person disclaims beneficial ownership of the shares held by Holiday Stationstores, Inc. (&quot;Holiday&quot;) and Holiday/GMTN Family LLC except to the extent of the reporting person's economic interest in these shares.</footnote>
        <footnote id="F3">Certain individuals, including the reporting person, transferred an aggregate of 1,042,174.926 shares of the Issuer's Common Stock to Holiday/GMTN Family LLC in exchange for the same number of units of Holiday/GMTN Family LLC. The aggregate number of shares transferred was incorrectly reported as 1,392,520.802 in a Form 4 filed by the reporting person on January 19, 2010.</footnote>
    </footnotes>

    <ownerSignature>
        <signatureName>/s/ Jonathan R. Zimmerman on behalf of Brian A. Erickson</signatureName>
        <signatureDate>2010-02-01</signatureDate>
    </ownerSignature>
</ownershipDocument>
