GMs will not deal with this situation. If Gil was placed into the FC chest, it is no longer the property of the person that placed it in there, it is the FC's property, same with items. There is zero obligation to give anything back to someone if they decide to leave. It's best to cover your butt, of course, and place it in your FC's guidelines (if you have any), but the GMs will not forcibly remove the gil from your company chest.