For really uncommon platforms, the only answer may be to build GIMP yourself. And there are other good reasons to build your own GIMP.
Perhaps you want the absolute latest and greatest version, to check out the hot new features. Maybe you want to have several versions installed at once. What if you're running an older OS, and the only GIMP available for it is too old? You might want to contribute to the GIMP project, or just read the code and play with it a bit to understand how it works. These are all good reasons.
In this appendix, you'll find
Getting the source
Notes on assembling dependencies
The three stages of building: configure, make, and make install
GIMP source usually comes packaged as a ...