You should check to make sure what run-level you are in after boot, and if gdm is being started for the run-level. You can do this manually by checking the symlinks in your /etc/rc*.d directories, where * is the number of the run-level.
First check to make sure you have a gdm init script, and run it to verify that it properly starts gdm:
$ ls -l /etc/init.d/ | grep gdm
$ sudo /etc/init.d/gdm start
If that works, then you can check the symlinks in the runlevel that you are in you can get you current runlevel with the 'runlevel' command. Then check for the correct symlink
$ ls -al /etc/rc3.d/ | grep gdm
If it's not there, create the symlink:
$ cd /etc/rc3.d/
$ sudo ln -s /etc/init.d/gdm
After rebooting, this start start the gdm init script. Make sure you are using the correct init script filename. I am using 'gdm' because I assume thats what it's called, but I could be wrong.