When selecting the AD Gateway from the list of AD Gateway Candidates, not all machines are listed.
The ICDM only displays a partial list of possible AD Gateway clients.
After selecting Add AD Gateway and then Select New AD or Existing AD Gateway, there will be an option to Search Devices.
When selecting Search Devices you can search by Device Group, Name, OS or OS Version.
For information on AD sync in ICDM please see the technical document Synchronizing Active Directory
For information on adding an AD gateway for Threat Defense for Active Directory please see the technical document Adding and updating an AD Gateway for a domain