GDM is the GNOME Display Manager, a graphical login program, that uses XWindow.
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.
To stop the gdm service, you can use the command
#invoke-rc.d stop
This command requires root privileges. Other possible options for the above command instead of 'stop' are --- 'start', 'restart', 'reload', 'force-reload' etc.,
You can also kill the X server (using CTRL + ALT + SUPR).
You can use gdm to load different Desktop environments and WindowManagers, using the session menu at the gdm login screen:
And login like a normal (login as root is forbidden by gdm by default).