Though this is a bad idea yes it is possible. The fastest method would be to do a Ctl+Alt+F1 and login as root. If there is no root password then login as a regular user and do the sudo su. After that you can use top to find and kill the xserver. Lastly run startx which should put you at a graphical desktop.
This is a very bad idea. If you need to run graphical applications to run as root and you do not want to mess with the gnome utility to do so then simply sudo su into a root shell from your terminal then run it from there. for example "/~> nautilus" this is how I manage files that need root permission and it works well.