The problem could be caused by a number of things, you should check the following:
1) Have a look for a file called "default-display-manager" in the /etc/X11 directory:
If you see the file "default-display-manager", then edit it by doing the following:
Make sure the following line is there:
If it is not, then add type the above line in, then save the file CTRL+O and then exit CTRL+X
2) Have a look in your /etc/init.d directory:
There should be a file called 'gdm', which is a script that is used to launch Gnome Display Manager.
If you don't have it, you will need to somehow acquire that file.
3) Have a look in your /etc/rc3.d directory. In there you should find a file (actually a symlink), such as S13gdm or similar number.
If not, then you might want to add a symlink to the /etc/init.d/gdm script file.
ln /etc/init.d/gdm /etc/rc3.d/S13gdm
Hope this helps.