When it comes to web design and creating web graphics, Photoshop is probably overkill for most of us.
Photoshop is a powerhouse that is used in all types of graphic work (print, photography, 3d etc ..) and so it has a lot of features that we humble web designer will not use. I think the point made that prospective employers will be looking for photoshop skills in their web designers, has some merit.
... That said, if you learn to use the Gimp you will be able to jump into Photoshop very quickly and so I don't see a problem in using the Gimp for web graphics work.
Stefan