If it were Gulpin I'd think it'll have to have really low stats and look kinda like a balloon which is running out of air or a small bean bag which is a lighter colour then Gulpin. C: Make it look more like a baby. :P
Yeah, that's exactly how I'd imagine it to look. XD We don't really have to worry about stats, because not much people are going to be using it anyway. It would be there only for the sake of it, haha. As for Koffing pre-evolution, I'm totally a-okay with that. Its body is already pretty big, so a smaller version of it would definitely seem believable.