Some found themselves in a situation where GDM no longer allowed #X11 sessions, but I'll explain what's going on.
At the moment, support for X11 sessions has been disabled (for testing reasons), but there is a flag available to put in the GDM config which is EnableXorg=True.
But why? There is the process of removing the #GNOME #Xorg session.
But don't worry, you can still enable X11 sessions, and it might be reverted to "True" by default later or with the new version.