<?xml version="1.0" encoding="UTF-8"?>

<!--XBRL Document Created with the Workiva Platform-->
<!--Copyright 2025 Workiva-->
<!--r:5a1bbe35-d56d-4cf6-9655-3940a4854422,g:a1f2e31b-47c9-4aca-a44c-9e3bd46d0b65-->
<link:linkbase xmlns:link="http://www.xbrl.org/2003/linkbase" xmlns:xlink="http://www.w3.org/1999/xlink" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xbrldt="http://xbrl.org/2005/xbrldt" xsi:schemaLocation="http://www.xbrl.org/2003/linkbase http://www.xbrl.org/2003/xbrl-linkbase-2003-12-31.xsd">
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/all" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#all"/>
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#hypercube-dimension"/>
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#dimension-default"/>
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#dimension-domain"/>
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/domain-member" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#domain-member"/>
  <link:roleRef roleURI="http://xbrl.sec.gov/ecd/role/PvpAdjustmentsOnly" xlink:type="simple" xlink:href="https://xbrl.sec.gov/ecd/2024/ecd-2024.xsd#PvpAdjustmentsOnly"/>
  <link:definitionLink xlink:role="http://xbrl.sec.gov/ecd/role/PvpAdjustmentsOnly" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_ecd_AllAdjToCompMember" xlink:href="https://xbrl.sec.gov/ecd/2024/ecd-2024.xsd#ecd_AllAdjToCompMember"/>
    <link:loc xlink:type="locator" xlink:label="loc_it_AggregateGrantDateFairValueOfOptionAwardAmountsReportedInSummaryCompensationTableMember_aa749a6c-ea99-466b-a02e-a776b42ad9ba" xlink:href="it-20250415.xsd#it_AggregateGrantDateFairValueOfOptionAwardAmountsReportedInSummaryCompensationTableMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ecd_AllAdjToCompMember" xlink:to="loc_it_AggregateGrantDateFairValueOfOptionAwardAmountsReportedInSummaryCompensationTableMember_aa749a6c-ea99-466b-a02e-a776b42ad9ba" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_it_EquityAwardsModifiedDuringTheYearMember_d31e13d6-39e0-43c2-ac15-a2a712c56bd9" xlink:href="it-20250415.xsd#it_EquityAwardsModifiedDuringTheYearMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ecd_AllAdjToCompMember" xlink:to="loc_it_EquityAwardsModifiedDuringTheYearMember_d31e13d6-39e0-43c2-ac15-a2a712c56bd9" xlink:type="arc" order="5"/>
  </link:definitionLink>
</link:linkbase>
