Anything is possible, as long as bitcoin and crypto currencies are concerned, and even judging from how old gold is, and how old bitcoin is, it is easy to tell that bitcoin is an asset that is on steroid looking at how far it has grown in just a few years of its existence.
Bitcoin market cap will surpass that of gold, but when this will happen is what I can possibly tell, since it's very hard to predict the future, and also, we should remember that, as far as bitcoin is already here, the growth of gold will be reduced significantly because, right now, gold may likely lose some of its investors to bitcoin, that is, some gold holders may at a point ape in on bitcoin, which simply means that gold may continue to lose some of, or a significant portion of its marketcap to bitcoin.
So, rest assured that bitcoin marketcap will one day catch up with that of gold, but when (like I said before) is what I can't predict.