your gdm.conf-custom has "SystemMenu=false", weird it didn't got listed by the grep. Does changing it to true and restarting gdm fixes the issue?
-- Shut Down and Restart buttons gone--hibernate instead https://launchpad.net/bugs/57548 -- desktop-bugs mailing list desktop-bugs@lists.ubuntu.com https://lists.ubuntu.com/mailman/listinfo/desktop-bugs
