You can do all the same stuff in the gimp, but it's just a little more laborious (or a lot more laborious if you're used to photoshop already I guess). The gimp lets you easily do all the same transformations, copy and reapply transformations, etc. It seems like the adjustment layer keeps the original image and the transformations to make it easier to repeatedly tinker with those adjustments and reapply them more quickly, though.