I'm sorry, I thought I clarified that you needed the developer tools installed.
To install the GIMP:
Open a terminal window and input ' /sw/bin/fink install system-xfree86' (no quotes on any of this). What this command does is installs a package which will check to see if there is a XWindows server installed. If you think you have already installed this package, run this command anyway, because if you have successfully installed it, it will not install a second time.
Then, when you see the command prompt again, input '/sw/bin/fink install gimp'.
This tells Fink to install the GIMP application. For me, anyway, this resulted in a fairly large (12MB) download, after which it ended up 'compiling' for almost 15 minutes. While it is downloading, you will probably see something which tells you how the download is progressing, and while it is compiling, you should see an 'endless' stream of text outputted into the terminal window. When it is done, you should see the command prompt again.
After this, open the X11 application and type 'gimp'. This should open the GIMP application, however, if it doesn't, then type '/sw/bin/gimp' which certainly should.
If you listen to the Fink people, you may experience 'strangeness' when using Fink if you have installed anything by the Open Darwin project. I don't have anything by Open Darwin, so I can't vouch for this, however, if you are encountering 'strangeness', this might be the reason.
I hope this helped you, but if you still need help, please post here, and I will try to help you out. You can also try the macosx.com IRC channel (see elsewhere on the site for more on this), or you can contact me on AIM using alexoreo, and I will walk you through it.