The users for the gdm user list are gathered by AccountsService. It will automatically hide system users (UID < 1000). To hide ordinary users from the login list create or edit a file named after the user to hide in /var/lib/AccountsService/users/ to contain at least:
Hide user from login list
The users for the gdm user list are gathered by AccountsService. It will automatically hide system users (UID < 1000). To hide ordinary users from the login list create or edit a file named after the user to hide in /var/lib/AccountsService/users/ to contain at least:
/var/lib/AccountsService/users/username
[User] SystemAccount=true