I have Snow Leopard 10.6.3 on a MacBook Pro. I had same problem. I followed directions at the end of this link:
http://www.strayprocess.com/blog/gimpshop-snow-leopard At the bottom of that page is a library.zip folder. Download it. In it are two files. Copy them into the lib folder of Gimpshop, replacing the one's in there.
To be explicit, that is, Control-click on the Gimpshop icon (or right click if you have a right clicker

, open package contents, then open resources, then lib. copy your new files into it and select Replace. that's it!
hope it helps.
Cosmotion