I disagree with the main point. IMO, it is fine the way it is now: it is NOT possible to steal GP. In a game where war is a great way to go (maybe, the most effective one) and let you obtain what you might have missed (conquering cities you haven't found yourself, obtaining wonders you haven't built yourself, etc), it's fine to have something that can be obtained ONLY if YOU produce it by YOURSELF. If you want GP, you must produce them, so you have to think at something else, apart of war. You cannot use your army to obtain them as well. I find it to be well balanced as it is now.