There are estimates of how much gold is left to be mined. The thing with gold is that if demand increases, so does mining (which means more supply). Bitcoin's supply increases at a known fixed rate, irregardless of demand. Plus there is the issue of gold on asteroids and other planets etc that will likely come onto the market in the future. Bitcoin doesn't have that problem either. Which is why bitcoin is and will be harder money than gold in the near future.