I recently installed the new Gnome 3.8 on my Ubuntu OS thinking it would only add the DE to my session manager at log in. I was wrong however. It actually changed the default to Gnome! I prefer Unity much more, but cannot change it back. How can I fix this?
If the problem is the session manager, you can switch using the command:
A dialogue will appear where you can chose between GDM and Lightdm.
The process is better explained here.