Oh, I just did a long post on how to install the GIMP using Fink, but it appears that you installed the OpenDarwin version instead. In this case, quoting from the OpenDarwin.org website, the command for running the GIMP is '/usr/local/bin/gimp-1.2 --no-shm --no-xshm'
This command must be entered into the X11 application, not the terminal, by the way.
Please let me know if this works for you.