Document IDs are used in SharePoint to facilitate document management.

For more information about Document IDs, see the Microsoft article Enable and configure unique Document IDs.

Document IDs are not supported when you migrate to an on-premises version of SharePoint. Even though ShareGate Migrate can sometimes migrate them, there is no guarantee it will work.

You cannot map Document IDs when you migrate content in Insane mode.

When you migrate in Insane Mode, SharePoint will always use the source's Document IDs at the destination (if the source is empty, the destination value will remain empty).

You can use Normal mode to map your Document IDs.