That would cause lots of trouble, but what about being able to buy from the store an item that is tradable ingame and is equivalent to a game time card? I think Eve Online offers something similar to that. That way people could in some sense "buy gils" but without creating inflation (the market would adapt to the supply of this kind of items). I bet there's people that would love to buy gold but don't because it isn't allowed, and many others like me would love to be able to pay the subscription with gils