Created a VM with 64bit 12.1 which did a smart install with KDE as the default desktop. Having done this a number of times, I know that in order to use Gnome, you have to log in and install the Gnome Desktop pattern. My problem is that I can’t see how to start a session with Gnome rather than KDE. In 11.x, there is a menu that lets you select between Gnome and KDE. The new login screen allows for only a username and password (as far as I can see), with no way to switch to Gnome.
So using the SUSE themed KDM login screen you have two buttons on the bottom left and the most left button seems to have three horizontal lines on it. If you select this button this is your session selector to pick between your loaded desktops in openSUSE 12.1.