A better source on the gold existing states:
At the end of 2014, there were 183,600 tonnes of stocks in existence above ground. If every single ounce of this gold were placed next to each other, the resulting cube of pure gold would only measure 21 metres in any direction.
http://www.gold.org/supply-and-demand/supply
183,600 Metric tonnes equals 202,384 short tons
That changes my estimate above a little, 10% easier to accomplish.
Random fact: I read that one near earth asteroid called 433Eros has been estimated to have 125,000 metric tons of gold (as well as other precious metals). I am betting that more commercial interest will be in mining asteroids than going to Mars. Of course it would crash the price of gold here.