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.
Adjustment layers are a key part of a non-destructive editing workflow. Without them, doing professional work like retouching is much more difficult. GIMP's issue is that they don't seem to care about power user workflows.
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.