Have you hand-edited your /etc/gdm/gdm.conf-custom file? I had and have left one section incomplete and had the exact same problem. When I deleted that file and did a "/etc/init.d/gdm restart" the shutdown and restart buttons reappeared.
You can also just reinstall the gdm package which will overwrite your gdm.conf-custom file with the default. -- GDM missing half of it's menu, no way to shutdown except with the command line https://launchpad.net/bugs/45320 -- desktop-bugs mailing list desktop-bugs@lists.ubuntu.com https://lists.ubuntu.com/mailman/listinfo/desktop-bugs
