People who want to see Gaster get brutalized have been a bit under-served SO
This isn’t a literal representation of what’s going on or anything, since these two are in a non-physical plane of existence at the moment. It’s just sort of a way to visualize it in understandable terms? So I wouldn’t take Gaster’s appearance or the monster thumping him around too literally.
Gaster (c) Undertale: not much fun being at the mercy of someone much stronger and cruler than you is it gaster