GNOME Bugzilla – Bug 774198
New groups not activated after log out and login
Last modified: 2016-11-10 14:03:56 UTC
When I add a user to a new group in Gnome3 and log out of Gnome and login again, the user is still not part of the group. After rebooting the computer the user is in the new group. I suggest that gdm somehow reloads the group information before logging a user in.
there's nothing gdm could do that it doesn't already. If you're having issues, it's probably with ncsd or sssd or ypbind whatever service you're using for nsswitch