GDM − The GNOME Display Manager
GDM is a replacement for XDM, the X Display Manager. Unlike its competitors (X3DM, KDM, WDM) GDM was written from scratch and does not contain any original XDM / X Consortium code. GDM runs and manages the X servers for both local and remote logins (using XDMCP). See http://www.jirka.org/gdm.html for more details.
gdm is just a script that runs the actual gdm-binary executable. gdm-stop is a script that stops the current running daemon immediately, gdm-restart restarts the current daemon immediately and gdm-safe-restart restarts the current daemon after everyone has logged out. gdmsetup is a graphical tool for easily changing the most commonly used options.
For full documentation see the GNOME help browser under the GNOME / System section.
thus also gdm-binary accept the following options:
Do not fork into the background
No console (local) servers to be run
Preserve LD_* variables
Print the GDM version
Print simple description of accepted options
gdmsetup accepts all standard GNOME options.
Configuration is done either by running gdmsetup or by editting the /etc/X11/gdm/gdm.conf (usually, could also be /etc/gdm/gdm.conf) file. The graphical tool does not support all the options possible so editting the configuration file is sometimes necessary.