Do not forget that such amount of gold would decrease the value of gold itself by a factor of 100 at least. Gold has value because its difficult and coslty to extract. With this new asteroid the value of gold would drop to new lows. What the man should do is to keep it hidden and lie about it, sell small amounts of gold everyday. Nobody would know the supply increased by that much until they all got ripped off by our man and his new found gold. The gold market is too small to absord such an amount of physical metal. If I was the man I would maximize my return but all the people who would buy my gold would end up with worthless metal.

Also if he found a gold asteroid its safe to assume he could find another one and drop the value of gold even more in the future.