A few distributions back the option to display a clock on the gnome-panel in GMT was removed, however to preserve backwards compatibility the option was left in which you could edit by hand or through gconf-editor. Since I have upgraded to Ubuntu 10.04 I've noticed that the option does not work any longer.
My method has been:
then under: / -> apps -> panel -> applets -> applet_N -> prefs
set: gmt_time = True
PS In writing this post I realize this is actually a Gnome issue and maybe I should go post on the Gnome forums...