Quoted from: http://forums.macosxhints.com/

Using OSX10.3.5, Gimp.app version 2.0

This might be an old thread, but I was searching for how to add fonts to Gimp and haven't come up with a lot. There is a fink package called applesystemfonts that might do the trick. When reading about this package it said that ttf fonts will be symlinked. So I tried it (symlinking) since I only needed one font in Gimp and it worked.

Via terminal cd to ~/.gimp-2.0/fonts

type: ln -s /Library/Fonts/fontinquestion.ttf ./fontinquestion.ttf

You can then refresh the font list via the font dialog in Gimp and it will show up (No need to restart Gimp or X11)