You can edit the gdm configuration file manually. Press Ctrl-Alt-F1 (or it might be Ctrl-Shift-Alt-F1), and you should be sent to a text-based login prompt.
My gdm configuration file is located at /etc/X11/gdm/gdm.conf
Another option might be to run startx after you have logged in. I don't know if the existing X server would interfere though. If you can get in X, then there's a program root can use to configure gdm graphically: gdmconfig
Again, on my system, gdmconfig is located at /usr/sbin/gdmconfig