Press Ctrl+Alt+F1 and login with your user, Then run:
sudo update-alternatives --config default-displaymanager
to select a different display manager. If you don’t have other, try to install xdm (sudo zypper in xdm) and then select it. Then reboot your machine (sudo reboot). If you can login but gnome can’t start, then press again Crtl+Alt+F1 login and run
sudo update-alternatives --config default-xsession.desktop
and select other desktop (IceWM), then reboot. Hope this help.