GDM allows 3 different display types.  First local static X servers.
These are always run, and when they die or are killed, they are
restarted.  GDM can run as many of these as needed.  GDM can also
manage servers on which it does not manage a login itself, thus
allowing GDM to be used when building X terminals.
Next GDM supports flexible or on demand servers.  These are run by
requesting one using the socket protocol.  There is a command,
gdmflexiserver, which can do this for the user.
For standard X servers the user must be logged in from a console, on
one of the servers that GDM has run.  This command can however also
launch nested Xnest servers which can be started
even from non-console logins.  This is generally done by running
gdmflexiserver -n.  These servers
are not restarted when the user session ends.
gdmflexiserver normally also locks the users
screen before running a new server with gnome-screensaver.
Last display type is the XDMCP remote displays that are described in
the next section.  Remote hosts can connect to GDM and present the
login screen if this is enabled.  Some things may be different for
these sessions, such as the Actions menu which allows you to shut down,
reboot, or configure GDM will not be shown.
