<?xml version="1.0"?>
<ownershipDocument>
    <schemaVersion>X0201</schemaVersion>

    <documentType>4</documentType>
    <periodOfReport>2003-11-03</periodOfReport>
    <notSubjectToSection16>0</notSubjectToSection16>
    <issuer>
        <issuerCik>0000040211</issuerCik>
        <issuerName>GATX CORP</issuerName>
        <issuerTradingSymbol>GMT</issuerTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001077650</rptOwnerCik>
            <rptOwnerName>DAMMEYER RODNEY F</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>4350 LAJOLLA VILLAGE DRIVE</rptOwnerStreet1>
            <rptOwnerStreet2>#980</rptOwnerStreet2>
            <rptOwnerCity>SAN DIEGO</rptOwnerCity>
            <rptOwnerState>CA</rptOwnerState>
            <rptOwnerZipCode>92122</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
            <reportingOwnerRelationship>
            <isDirector>1</isDirector>
            <isOfficer>0</isOfficer>
            <isTenPercentOwner>0</isTenPercentOwner>
            <isOther>0</isOther>
        </reportingOwnerRelationship>
    </reportingOwner>
  <nonDerivativeTable>
   <nonDerivativeTransaction>
      <securityTitle>
          <value>Common Stock</value>
      </securityTitle>
      <transactionDate>
          <value>2003-11-03</value>
      </transactionDate>
      <transactionCoding>
          <transactionFormType>4</transactionFormType>
          <transactionCode>A</transactionCode>
          <equitySwapInvolved>0</equitySwapInvolved>
      </transactionCoding>
      <transactionTimeliness>
      <value />
      </transactionTimeliness>
      <transactionAmounts>
          <transactionShares>
              <value>630.39</value>
              <footnoteId id="F1" />
          </transactionShares>
          <transactionPricePerShare>
              <value>22.605</value>
          </transactionPricePerShare>
          <transactionAcquiredDisposedCode>
              <value>A</value>
          </transactionAcquiredDisposedCode>
      </transactionAmounts>
      <postTransactionAmounts>
          <sharesOwnedFollowingTransaction>
              <value>5525.87</value>
              <footnoteId id="F2" />
          </sharesOwnedFollowingTransaction>
      </postTransactionAmounts>
      <ownershipNature>
          <directOrIndirectOwnership>
              <value>D</value>
          </directOrIndirectOwnership>
      </ownershipNature>
   </nonDerivativeTransaction>
  </nonDerivativeTable>
  <derivativeTable>
  </derivativeTable>
<footnotes>
   <footnote id="F1">Includes that portion of the quarterly payment of the annual directors' fee (630.3915 shares) credited to an account established for each director under the GATX Directors' Phantom Stock Plan ("Phantom Stock Plan") which in each case will be settled in common stock upon the expiration of the director's service on the Board.</footnote>
   <footnote id="F2">Includes 82.7357 shares acquired pursuant to a reinvestment feature of the GATX Phantom Stock Plan.</footnote>
</footnotes>
<remarks></remarks>    <ownerSignature>
       <signatureName>Lisa M. Ibarra, by Power of Attorney on behalf of Rod F. Dam</signatureName>
       <signatureDate>2003-11-03</signatureDate>
    </ownerSignature>
</ownershipDocument>

