Gdm

GDM (GNOME Display Manager) is a login manager that is designed to be consistent with the GNOME system. It is similar to the traditional login manger, xdm, though shares no code with it.

The manager can be restarted (disrupting your session), by /usr/sbin/gdm-restart or, if you do not wish to disrupt your session, /usr/sbin/gdm-safe-restart