GDM gone missing

[Michael Wardle]

Try running these commands to understand the problem:
user at ubuntu:~$ which gdm
user at ubuntu:~$ sudo which gdm
user at ubuntu:~$ dpkg -L gdm | grep "gdm$"

(assuming your shell's prompt is "user at ubuntu:~$ ")

You should notice that gdm is installed in /usr/sbin/, which is only in 
root's command search PATH, and not in a normal user's PATH.  This means 
you'll have to type the full path "/usr/sbin/gdm" if you're not root.

The best way to start it is:
user at ubuntu:~$ sudo /usr/sbin/invoke-rc.d gdm restart

or the same thing in a non-Debian-specific way:
user at ubuntu:~$ sudo /etc/init.d/gdm restart

You can also verify that gdm is properly installed by running:
user at ubuntu:~$ dpkg -l gdm

you should see a line something like:
ii  gdm             2.8.0.1-0ubuntu4               GNOME Display Manager

if the first two letters aren't "i", you have a problem.  See the 
dpkg(1) and dpkg-query(1) manual entries for more information.

You can also get more information with:
user at ubuntu:~$ dpkg -s gdm

Hope this helps