Image editor which can handle layer groups
There are two "great" image programs for the Mac at this time, so if Gimp doesn't do it your only valid alternatives are Pixelmator and Acorn.

I have an old version of Pixelmator that doesn't do Layer Groups (1.x), from what I can see, 2.x doesn't do it either. I have Acorn and although it mentions PSD compatibility (and Layers) I don't have a .PSD file (because I don't have Adobe Photoshop).

So you can download a trial version (of Acorn at least) and give it a try.