There are real practical problems with gold and changing economies. What happens when your population goes up - you need more gold. Or economic growth. Similarly if gold is discovered like Spain's Latin American winnings it causes inflation.
Gold being poorly divisible also makes it very impractical for day to day coinage of a growing population. Eventually, the coins start getting so tiny as to be impossible to work with. I believe that's a big reason why we started using silver and copper for smaller denominations at some point.