Please, tell me what X11 is then, and how to use it...
X11.app is Apples implementation, which is much better than any other implementation for MacOS X.
The X Server is what actually displays the program, while the X Client is the Program itself. X11.app is an X Server, GIMP is an X Client. You can display a Client on a remote computer on a Local Server;
For example, you could run Wolfenstein on an x86 Linux machine, and display the output on your Mac, but this would be really slow, since the Display would have to be passed over the network, and the Client hasn't been optimized for your X Server. I personally wouldn't bother.