I find it unlikely, if possible, that Gold buried the box.
But I really think it is evident that neither he nor Regina can do magic like they could in FTL. I think they can only use magical artifacts they have, such as Regina with Graham’s heart. I think that is why Gold is collecting artifacts, because they have power, and he doesn’t. I think if either he or Regina could do magic to any appreciable level, we’d have seen some indication by now.
Even so, I don’t think this is something Gold would do. He works in elaborate schemes and debt to control people, to make people think they are doing what they want, when they are really doing what he needs. Regina is the one who uses force and more direct forms of blackmail, extortion, and threats. If Gold were stealing them, it would be just to prevent Regina using them, and that doesn’t work as a narrative, in my opinion.