If you talking about how do you log into the window manager (gnome) in as root in ubuntu, you can't. Well, in reality, you can, but it takes a bit of hacking. The Ubuntu team did that as a security measure, I found it quite annoying at first, but I got used to it and it doesnt bother me anymore.
You can just use su, or sudo and do all of the same things as if you were logged in as root to gdm.