To be fair, I don't think Stallman is against buying or selling software, I think he just feels that the source code should be provided with it (and that you can share it afterwards...)
Under the GPL you could have bought your copy of the game knowing that you could retrieve the source code, make the changes necessary for the software to function and distribute the changes under the same license. Or just retrieve someone else's changes to that end.
Might seem very impractical for a commercial game, but Id software released the source code of a bunch of their engines within a few years of the initial release of the games based on them, meaning you would only need to supply the still-proprietary game data to have a functional game. As a result there are still rather recent commercial releases based on free software derivatives of these engines.