X Windowing Layer
GIMP uses a separate windowing layer, the X11 windowing protocol, on Mac OS X, which must be installed first. The XDarwin project provides this functionality. Apple used to provide their own version of this windowing layer, based on XFree86. Mac OS X 10.5 (Leopard) now ships with an X11.app based on the X.Org 7.2 release.