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

    <schemaVersion>X0202</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2007-05-30</periodOfReport>

    <issuer>
        <issuerCik>0000749251</issuerCik>
        <issuerName>GARTNER INC</issuerName>
        <issuerTradingSymbol>IT</issuerTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001198849</rptOwnerCik>
            <rptOwnerName>HALL EUGENE A</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>56 TOP GALLANT ROAD</rptOwnerStreet1>
            <rptOwnerStreet2>P.O. BOX 10212</rptOwnerStreet2>
            <rptOwnerCity>STAMFORD</rptOwnerCity>
            <rptOwnerState>CT</rptOwnerState>
            <rptOwnerZipCode>06904-2212</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>1</isDirector>
            <isOfficer>1</isOfficer>
            <officerTitle>CEO</officerTitle>
        </reportingOwnerRelationship>
    </reportingOwner>

    <nonDerivativeTable>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <transactionDate>
                <value>2007-05-30</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>F</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionTimeliness></transactionTimeliness>
            <transactionAmounts>
                <transactionShares>
                    <value>124350</value>
                    <footnoteId id="F1"/>
                </transactionShares>
                <transactionPricePerShare>
                    <value>0</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>D</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>429759</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
    </nonDerivativeTable>

    <footnotes>
        <footnote id="F1">Represents shares withheld from the released restricted stock for the payment of applicable income and payroll withholding taxes due on release.</footnote>
    </footnotes>

    <remarks>On November 9, 2005, Gartner and Eugene A. Hall, its chief executive officer, agreed to cancel Mr. Hall?s existing 500,000 share restricted stock award and replace it with a new award for the same amount of shares and on similar terms. This was done for tax reasons and the number of shares of restricted stock issued to Mr.Hall remains unchanged. By issuing the restricted stock award under its stockholder approved 2003 Long Term Incentive Plan, Gartner will be able to take a tax deduction when and if the restrictions lapse on the restricted stock award. Gartner would not have been able to take advantage of this tax deduction on the award in its current form because the award had been made as an inducement grant, and consequently was not issued pursuant to a stockholder approved plan. Gartner and Mr.Hall have entered into (i) a Termination of Restricted Stock Agreement to cancel the original award of 500,000 shares of restricted stock which was made on October 15, 2004; and (ii) a Restricted Stock Agreement which makes a new grant to him of 500,000 shares of restricted stock under Gartner?s 2003 Long Term Incentive Plan.

Similarly to the grant being cancelled, the restrictions on this new grant lapse as to (i) 300,000 shares when the Company?s Common Stock trades at an average price of $20 or more for sixty (60) consecutive trading days, (ii) 100,000 shares when the Company?s Common Stock trades at an average price of $25 or more for sixty (60) consecutive trading days, and (iii) 100,000 shares when the Company?s common stock trades at an average price of $30 or more for sixty (60) consecutive trading days, subject to Mr.Hall?s continued employment with the Company through each such date. Notwithstanding the preceding sentence, all restrictions shall lapse in full upon a change in control.</remarks>

    <ownerSignature>
        <signatureName>/s/ Eugene A. Hall</signatureName>
        <signatureDate>2007-05-31</signatureDate>
    </ownerSignature>
</ownershipDocument>
