Gas is a pretty icky liquid. :eek:

The vinyl and plastic coating and the thin woods used in guitar cases are all porous, they'll absorb liquids (including the stinky ones) very easily. Unless your case was waterproofed or rain-x'ed or something, you're probably going to be stuck with either buying a new case or wearing a closthespin over your nose to avoid the gas smell on it.

Ummmm... you might try a good dousing of Oxy-clean or something similar, but that's a long shot. :(