I've started carving into drywall, and am liking this better than the gypsum cement I was carving into (ultracal 30) as I needed hammer and chisel for that. The drywall is much coarser and easier to remove material by hand.
I was also able to go through a progression of sandpaper and get it feeling smooth. Two big issues are the paper easily pulls up from the sanding process, and edges of it get sloppy. And the second issue is that after carving, the drywall becomes very delicate as there is less material on the paper backing. So I think I need to mount the drywall to plywood before carving.
I also have to figure out how to seal the paper so it doesn't get gross when sanding, or maybe find a way to remove it cleanly from the gypsum. If I can do that, then I don't need to create my own gypsum panels, I can just buy drywall, which would be cool as I love the idea of transforming the exact material used to build the walls that we live with everyday.
Looking forward to getting dirty after writing this :)