This came up recently, and I saw a couple of vague responses. So if this is still relevant to anyone, here's how you can make it possible to call shutdown from gdm.
* Grab root via your favorite method (su or sudo) and open /etc/gdm/gdm.conf in your favorite editor. For me, this looks like this: $ sudo xemacs /etc/gdm/gdm.conf * Find the [greeter] section of the conf file, and specifially the SystemMenu line. In my particular gdm.conf file, the second occurance of the string 'SystemMenu' is the line you want. It should look something like this: SystemMenu=0 * Edit SystemMenu to equal 1. Save the file, and log out. Next time gdm comes up, it should have a 'System' pull down menu with 'Reboot' and 'Shutdown' commands available. Just because this works for me does not guarantee success for you.
