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?
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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
The X11 Gimp is actually quite misbehaved on OS X, to the point of being almost totally unusable. But despair not; there is a native Gimp version that doesn't require X11. You can find it on the Gimp download page.
Here http://gimp.lisanet.de/Website/Download.html you can download a native GIMP 2.8.2 and also the docs. No X11 is required.