1. Restarting GDM

GDM is the most common X display manager for the linux distributions.
It's very similar to XDM, and the GDM instance is used to manage 
severl virtual terminals, both text and X.

If GDM dies then the instance can be restarted with

ctrl+alt+backpace

Otherwise, to restart GDM, open a virtual console (e.g., ctrl+alt+F1)
and grep for the gdm(1) instance:

bash# ps -elf| egrep gdm | egrep -v grep

Once you have the gdm pid, send a SIGHUP (signal #1) to the process.
This needs to be done as root or as the user that owns the process:

bash# kill -1 29999

2. Starting multiple displays under GDM

By default, GDM starts a single virtual X session on :0, which is 
accessed via ctrl+alt+F7. There are 6 virtual consoles (text) started 
as well, these are ctrl+alt+F1 through ctrl+alt+F6. 

To start more than one virtual X terminal session, e.g., display :1 
accessed with ctrl+alt+F8, display :2 accessed with ctrl+alt+F9 and etc,
edit your gdm(1) configuration, which is typically in /etc/X11/gdm/gdm.conf,
and set the following options:

Note gdm.conf locations:
    FC4: /etc/X11/gdm/gdm.conf 
    FC5: /usr/share/gdm/defaults.conf

[daemon]
FlexibleXServers=5  # Not completely sure that this one is 
                    # related, but it "soundes" like it is.
                    # 5 is the default on my debian install anyway.

FirstVT=7           # This would be correct on a "traditional" distro 
                    # install, adjust for yours
VTAllocation=true

[servers]
0=Standard          # This is F7 in our "traditional" setup
1=Standard          # Now we've got a second x session, this one's on F8
2=Standard          # No. 3, it's on f9, and so on.

Next restart gdm(1) and that's it.

bash# gdm-safe-restart

3. GDM Theme Tester

GDM is supplied with a shell script which is useful in testing GDM themes.
It starts an Xnest session with the named theme. This script can run in
non-priviledged mode, but requires that the X11 library for Xnest is
installed. E.g., to test the theme "mono-metal"

bash$ gdmthemetester flexi mono-metal

The second argument is called the "environment" and supported values are
"console", "flexi", "flexi-remote", "console-timed" and "xdmcp".

4. Start new nested GDM Login

It's possible to launch new gdm login sessions within the current session
by using the GDM Flexiserver with the Xnest option. These nested
subsessions can be started as a non-priviledged user. Note, the X11 
Xnest libaries must be installed.

bash$ gdmflexiserver  --xnest

By default, the flexiserver will start the new gdm login manager within
an Xnest server window that is 3/4 of the size of the root window. This
behaviour can be changed by specifying a (standard X11) geometry or by
setting the XNESTSIZE variable.