Using CMD instead of Control in GIMP
It is possible to use CMD instead of Control for keyboard shortcuts. Just follow these instructions while GIMP and X11 are closed :
Create a .Xmodmap file in your home-folder containing the following:
keycode 63 = Control_L
keycode 67 = Control_L
add control = Control_L
Then open X11 and change its preferences like this :
Quick how-to if you're not sure how to do that
Open a programmer's text editor like Smultron. Paste the lines mentionned above in it. If you copied and pasted from your browser, it may have messed up line endings so be sure to fix them (in smultron you can fix that in the text menu - select Unix line endings).
Now save the file. Navigate to your home folder (the one with a house icon, you can find it in /Macintosh HD/Users/your_user_name/)
Name the file .Xmodmap then save it. It will give you a warning about files beginning with ".". Don't cancel, save anyway.
Now open GIMP. CMD+C and all the stuff should work.