open a terminal window
Code:
su -
(root password)
gimp
You won't have your settings or plugins, etc... that you created for your normal user unless you copy your ~/.gimp-2.7 folder over to /root/.gimp-2.7
The folder can be ~/.gimp-2.6 ~/.gimp-2.7 or ~/.gimp-2.8 depending upon the version of gimp you have installed.
Edit:
There really is no reason you need to run gimp as root, though. It can be dangerous running applications as root when you don't need to.