GDM themes are stored in /opt/gnome/share/gdm/themes. They can tested with a command like `gdmthemetester console GDM-SuSE'. `console' refers to the method of connection. New themes can be found in www.gnome-look.org.
The graphical configuration tool is not easy to access. As root, change to directory /opt/gnome/sbin/, then run `./gdmsetup'. gdmsetup can also be used to change/remove the `Login a user automatically on first bootup'.