open a terminal window
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.
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.