I moved to Photoshop from Corel, but I'd imagine that my reasons for doing so would be the same as for a Gimp user.

Photoshop is more widely used. As such, it is easier to share documents and resources with other artists, education is more common and deeper, and more extensions are available, although Gimp's open source nature means that it's catching up fast. There were a couple of features in Corel that I miss, and I know there are a couple in Gimp that I envy (the aforementioned bump mapping and image hoses, in particular). Those things weren't quite enough to prevent me from moving to PS, though.

It bears mentioning, however, that I am seeking a job as a commercial artist now, so my need to be compatible with other artists is much greater than it would be if I were still a hobbyist.