GDM is the GNOME Display Manager, a graphical login program, which provides a simpler alternative display manager for the X Window System's XDM. GDM is released under the GNU General Public License (GPL). The X Window System by default uses the XDM display manager. However, resolving XDM configuration issues typically involves editing a configuration file. GDM allows users to customize or troubleshoot settings without having to resort to a command line. Users can pick their session type on a per-login basis. GDM also features easy customization with themes.