GDM is the GNOME Display Manager, it is the little program that runs in the background, runs your X sessions, presents you with a login box and then tells you to bug off because you forgot your password. It does pretty much everything that you would want to use xdm for, but does not involve as much crack. It does not use any code from xdm. It 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).
Those who have contributed to GDM can be viewed in the AUTHORS file in the gdm SVN module.