The Gnome Display Manager (GDM) is a display manager that implements all 
significant features required for managing local and remote displays. 
For full information about GDM features, refer to the documentation.

The Sessions that gdm uses are installed in /etc/X11/gdm/Sessions.
Several example files are included for blackbox, fluxbox, fvwm2,
kde, twm, windowmaker, and xfce.

If you want xscreensaver to recognize gdm and add a "New Login" button
to the unlock screen, add this line to your ~/.xscreensaver file:
  newLoginCommand:       gdmflexiserver -ls

This requires libgnomecanvas.