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

    <schemaVersion>X0306</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2016-02-01</periodOfReport>

    <notSubjectToSection16>0</notSubjectToSection16>

    <issuer>
        <issuerCik>0000944480</issuerCik>
        <issuerName>GSE SYSTEMS INC</issuerName>
        <issuerTradingSymbol>GVP</issuerTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001665849</rptOwnerCik>
            <rptOwnerName>Pugh Daniel</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>C/O GSE SYSTEMS, INC.</rptOwnerStreet1>
            <rptOwnerStreet2>1332 LONDONTOWN BLVD</rptOwnerStreet2>
            <rptOwnerCity>SYKESVILLE</rptOwnerCity>
            <rptOwnerState>MD</rptOwnerState>
            <rptOwnerZipCode>21784</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>0</isDirector>
            <isOfficer>1</isOfficer>
            <isTenPercentOwner>0</isTenPercentOwner>
            <isOther>0</isOther>
            <officerTitle>SVP, General Counsel &amp; RMO</officerTitle>
            <otherText></otherText>
        </reportingOwnerRelationship>
    </reportingOwner>

    <nonDerivativeTable>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <transactionDate>
                <value>2016-02-01</value>
            </transactionDate>
            <deemedExecutionDate></deemedExecutionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>A</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>60000</value>
                    <footnoteId id="F1"/>
                </transactionShares>
                <transactionPricePerShare>
                    <value>0</value>
                    <footnoteId id="F2"/>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>60000</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
    </nonDerivativeTable>

    <derivativeTable></derivativeTable>

    <footnotes>
        <footnote id="F1">(1)	These shares of common stock represent restricted share units granted pursuant to a restricted share unit agreement between Mr. Pugh and the Issuer. The restricted share units awarded will vest in installments of (a) 14,400 shares when the volume weighted average price of the common stock exceeds $2.50 for a 90 consecutive day trading period, (b) 14,400 shares when the volume weighted average price of the common stock exceeds $3.25 for a 90 consecutive day trading period, (c) 14,400 shares when the volume weighted average price of the common stock exceeds $4.25 for a 90 consecutive day trading period, and (d) 16,800 shares when the volume weighted average price of the common stock exceeds $6.00 for a 90 consecutive day trading period.</footnote>
        <footnote id="F2">These restricted share units were granted to Mr. Pugh for his services as Senior Vice President, General Counsel &amp; Risk Management Officer of the Issuer.</footnote>
    </footnotes>

    <remarks></remarks>

    <ownerSignature>
        <signatureName>/s/ Daniel Warren Pugh</signatureName>
        <signatureDate>2016-02-05</signatureDate>
    </ownerSignature>
</ownershipDocument>
