The gksu dependency in gdm ebuild should be optional instead of hardcoded. Gdm compiles and works well without gksu, and if you are using gdm with any other WM/desktop than Gnome, gksu and its dependencies are just unnecessary cruft. The proposed patch for ebuild is enclosed, I added a 'gnome' useflag to pull gksu when needed.
Created attachment 167700 [details, diff] patch for gdm-2.20.8
Please have a look at the description of the package: "GNOME Display Manager" gksu is here to provide a nice way to get access to gdmsetup. It could be made desktop aware like in gparted but I really don't see any interest in this. kde has kdm, gnome as gdm and if you run xfce, you most probably have most deps already...
wontfix per comment #2