Gold Coins can be found lying around in the Bjornholm map in Farming Simulator 15. There are exactly 100 coins on the map, which can be collected by walking over them, similarly to collecting eggs. After you collect 10 coins, the other 90 coins will show up on your map and minimap as fairly large gold dots.
When all the coins are collected you will get a message telling you "They're completely worthless, time to throw them into a ditch", they can be thrown into a wishing well, located near field 35 and 40, at the end of a dirt road in the very corner of the map. When you do so, your silos will immediately be filled with 200,000 liters of each commodity.