I suspect that photoshop has niche tools that gimp does not have, but really, to my knowledge gimp can do anything photoshop can. I've observed the use of photoshop before, but I've never personally used it, so I guess I can't rely on personal experience for the comparison... That being said, I've seen no reason to 'switch' to photoshop; gimp has done anything I've wanted it to do....
I've used it for scaling, converting images, effects, adding shadows, creating textures, creating text boxes for videos, adding laser fire to video, making animated images... advanced image editing, logos, hell, I've even made business cards with gimp. I edit all camera photos with gimp and do not consider myself an expert by any means. There are craploads of user plugins, and you can use 'photoshop' brushes. The choice of brush is one of the biggest components for an artist, and well.... you can use whatever you want.
Any society that would give up a little liberty to gain a little security will deserve neither and lose both.
The reasonable man adapts himself to the world; the unreasonable one persists in trying to adapt the world to himself. Therefore all progress depends on the unreasonable man.
-George Bernard Shaw