To make it easier to run GIMP from X11, click Customize, then click Add Item. Put GIMP in the Name column, and in the Command column, enter the path to the actual GIMP executable, which is: /Applications/Gimp.app/Contents/Resources/bin/gimp. (If you installed it elsewhere, you'll need to change the portion of the path to the left of "Gimp.app".) In the Shortcut column, press the letter that you'll use with Command to launch the program -- I used 'g.'
So now, whenever X11 is active, I can start GIMP by pressing Command-G.

