On Friday, I spent the majority of the day banging my head against my laptop.
I tried several different ways to install the GIMP (and even GIMPshop), but it kept failing every time I tried to run it.
Eventually, I saw the cautionary warning on the site that hosts pre-built Mac OS X versions of the GIMP.
I will repeat the cautions here: The X11 package you can find on a OS X 10.5.0 install disk is too old to run GIMP (it will crash), and it can’t be updated through software update.
There appears to be several different ways to address the problem, but the one that worked for me was to download and install the latest version of the XQuartz project.