gdm.conf should only handle the login manager for your desktop, so the first thing to try would be to simply boot into runlevel 3 before the login manager starts and then edit the file from there using your favored text editor.
If my memory serves me you should be able to just type 3 at the grub screen and that will force the machine to boot to runlevel 3 as opposed to a higher runlevel where the login manager would start.
If that doesn't seem to be working the next thing I would suggest is burning a quick livecd (or usbkey) from another machine and then simply booting onto it and editing the file from there.
I grabbed the default gdm.conf for SUSE 10
you can see it here:
try sticking it on a usb key or cd, it's rather large.
If you are unsure of what to edit and just want the defaults, I would suggest figuring out what specific version of SUSE you are using, and then hopping onto the official IRC channel and asking someone for a copy.
Hope that helps,