Gdm provides the equivalent of a "login:" prompt for x displays- it pops up a login window and starts an x session.

it provides all the functionality of xdm, including xdmcp support for managing remote displays.

the greeting window is written using the gnome libraries and hence looks like a gnome application- even to the extent of supporting themes! by default, the greeter is run as an unprivileged user for security.

this is the legacy 2.20.x series which does not depend on gnome-session.