As per the NXT design, we consider username as the unique identifier by default in the SFDC connector job. Now -
1. Create a salesforce connection in Gainsight and complete the user Sync. This will create the user 1 in Gainsight.
2. Connect Gainsight to another environment/sandbox with similar data this will create another User 2 in Gainsight.
3. Now since there are 2 users with same Id.
4. Make User 1 as inactive and assign all the permissions to User 2.
5. User 2 in salesforce will see a blank screen.
Workaround is to deactivate the User 2 and assign the right permission for User 1.
This is not acceptable approach and would need a feature enhancement for the current functionality.