Gimpshop is a modification of The Gimp which includes some small (but nevertheless important) changes:

If you’ve never used Photoshop before, you may not appreciate my GIMPshop hack. What I’ve done is renamed and reorganized GIMP’s tools, options, windows, and menus to closely resemble Adobe Photoshop’s menu structure and naming conventions. Many of the menu options and even whole menus were recreated to faithfully reproduce a Photoshop-like experience. After running my GIMPshop hack, you’ll find that Photoshop and the GIMP are strikingly similar.

I’m using Gimpshop on my Linux Machine at work (as Linux lacks an official Photoshop Release – and doesn’t work smooth enough), and apart from the usual Gimp troubles, regarding selections and movement of layers, it’s working quite well.
There’s a Mac OS X Version too, so you might just as well give it a shot and see if it suites your needs – if you don’t have Photoshop, that is
GimpShop