I recently upgraded to Mountain Lion and tried to open GIMP. To my dismay my computer asked me where X11 was.
How do I find X11 and run GIMP?
Apple decided not to include X11 in Mountain Lion: Apple Support Article
But there is now a native version of GIMP for mountain lion which requires no X11: Gimp Downloads
However, if you need to run the old version of GIMP you can download xquartz as mentioned in the apple support document linked above.
When you try to run GIMP again, you can find Xquartz in