Here's how you can run the GIMP and other X11 applications full screen in Mac OS X Lion. 1. Use Mission Control to create a new desktop space for your X11 applications to run on. 2. Switch to your new space. 3. Hit Command+Space and type X11 then hit Return. 4. Go to your Dock and right click the X11 icon then choose Options and then This Desktop. This will make all X11 applications run on your new space. 5. Go to the Preferences menu for X11 then click the Output tab. 6. Check the Full-Screen Mode and Allow Menu Bar Access in Full Screen Mode boxes. Now all of your X11 applications will run full screen in Mac OS X Lion.
For more, see the original article at the link below.
Run The Gimp (Or Other X11 Apps) Full Screen [OS X Tips] | Cult of Mac