Over the long term, it is impossible to peg Bitcoin to gold. In 2140 when there are no more bitcoins to mine people will still be digging up gold from the ground. There would be a net imbalance that can only be maintained  by shorting bitcoins or buying up all the excess gold being mined. Since bitcoins cannot be shorted indefinitely somebody would have to keep spending fiat to buy up gold to keep it scarce. Why would they do that?