GDM is the GNOME Display Manager, it is the little proggie that runs in the background, runs your X sessions, presents you with a login box and then tells you to piss off because you forgot your password. It does pretty much everything that you would want to use xdm for, but doesn't involve as much crack. It doesn't use any code from xdm, and has a more paranoid and safer design overall. It also includes many features over xdm, the biggest one of which is that it is more user friendly, even if your X setup is failing. The goal is that users should never, ever have to use the command line to customize or troubleshoot gdm. It of course supports xdmcp, and in fact extends xdmcp a little bit in places where I thought xdm was lacking (but is still compatible with xdm's xdmcp).