The subject is almost true. I wanted to try gdm but it will not run when I boot:

I just get the message that Slim will not be started because it is not the default window manager.

Here is what I have done thus far:
apt-get install gdm (I did get the dpkg-reconfigure as part of the install)
dpkg-reconfigure gdm (since it did not work)
Then I checked /etc/X11/default-display-manager It shows /usr/bin/gdm..... However"which gdm" shows /usr/sbin/gdm)

changed default-display-manager to /usr/sbin/gdm (still does not work)

Now for the fun part. I can get to the console on boot... login as root and run /etc/init.d/gdm start and all is well. It works perfectly.

So in summary gdm works if I run it after booting but not as a part of booting up.

P.S. I have done an apt-get purge gdm and a reinstall to no effect.

Thanks for any help

Oh yeah I can also run kdm or go back to SLim with no problems

There was a bug in gdm from Sid repos, but that is supposed to be fixed.

Thanks