Gold is a virtual currency in this game and soon, not only in this game. We can sell gold for 5 Euros each or buy it for 6.5 Euros each from guys named re-sellers (normal players just like we are, but who have bought a concession to trade gold).
In trading we have an amount of gold that we can check on the upper part of our screens; right now as you can see it's 227g. The amount is getting higher because players continue to mine gold; gold trades are only between players.
For gold you can buy anything you want in this game as well like for regular resources: Vegetables, Meat, Wood, Iron, Stone or Clay. Gold is much more valuable than regular resources so it's logical that we can buy more for it. For example: 1 gold = 33.000 resources (if we count 0,0003 gold for x1 resource). At the beginning of the game 1 gold for 6,5 euro will help you a lot and let you build your city all the time without any stops, but you should get some more gold than only 1 to build your city properly and then make money out of it. Gold is a main resource in the game and a reason that game have been made. People get money by mining gold and selling it and company gets gold from taxes. For gold we can buy: workers, resources, shares, private countries (including islands), referrals, army, real gold bars. As a person and a friend to everybody, my advice is: BUY GOLD TO GET MORE GOLD.