Assuming that MacPorts is already installed and works O.K. follow these steps:
You will end up with The Gimp under the Applications/MacPorts folder and of course you can launch it from the CLI too, the program will look and behave as any other OS X application.
Here’s an screen-shoot (Gimp is 2.8.6 version):
Of course there are other ways to get The Gimp running under OS X… still the MacPorts allows you to maintain it very easily. By the way, there’s also Homebrew (and Fink for what is worth) which is another way to bring your favorite FLOSS into OS X, sadly at the time I wrote this The Gimp was not in the Homebrew tree.