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

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).