The GDM package provides quite a few different commands.
First command is the gdm command which is really just a script which runs the gdm-binary. If you really need to set some environment before launching GDM, you can do it in this file. This command accepts several command line parameters.
gdm Command Line Options
Gives a brief overview of the command line options.
If this option is specified, then gdm does not fork into the background when run. You can use just a single dash with this option to preserve compatibility with XDM.
Tell the daemon that it should not run anything on the console. This means that none of the local servers from the [servers] section will be run, and the console will not be used for communicating errors to the user. An empty [servers] section automatically implies this option.
When clearing the environment internally, preserve all variables starting with LD_. This is mostly for debugging purposes.
Next are the two greeters, gdmlogin which is the Standard Greeter, and gdmgreeter which is the Graphical Greeter. There is also the gdmchooser which is the XDMCP chooser application. These should all be run only from GDM and not by itself, although gdmchooser could theoretically be run as a standalone application which prints the chosen host on the standard output.
There is also the setup program which is the gdmsetup command. Normally on systems that support the PAM userhelper, this is setup such that when you run gdmsetup as an ordinary user, it will first ask you for your root password.
There is also the user picture setup program, gdmphotosetup. This is run by users to setup their face browser picture.
There are also the commands gdm-stop, gdm-restart and gdm-safe-restart which stop, restart, or restart the GDM daemon after all users have logged out, respectively. These are installed in the <sbin>/ directory.
Finally there is the gdmflexiserver command which runs flexible (on demand) X servers.
There are some extra commands provided as well. First is the gdmXnestchooser command. This command can run Xnest with all the right options and automatically getting the correct display number and setting up access, and runs the server as an indirect query to the local host by default. This way you get an XDMCP chooser provided by your machine. You can also supply as an argument the hostname whose chooser should be displayed, so gdmXnestchooser somehost will run the XDMCP chooser from host somehost inside an Xnest. You can make this command do a direct query instead by passing the -d option as well.
Also useful is the gdmXnest command which is just for more easily running Xnest on your machine. It will print out the display setting on standard output that you can use to connect to this server. This is useful mostly for developers who perhaps wish to test their apps running on a completely separate server.
For Graphical Greeter theme developers, there is the gdmthemetester command which can run the Graphical Greeter in a simulated debug environment for any selected theme. Run the command with no options to get help.