It does say "for 10.3/10.4" on their web site. That might be a hint.
I'd just go to http://www.gimp.org and get the "real" Gimp. Gimpshop is a modified version of Gimp that is always several version behind Gimp. I've had to move on the Photoshop because of the color management and "deep color" issues but if you can use Gimp in your work, it is great to use open source software.