Gold is the premium currency of Standoff 2 and can only be earned by purchasing it for real money or selling items in the Marketplace. It can also be used to buy items from the Marketplace and buy premium skin cases.
The price of 100 Gold is 1.99€ [not taking into account any discount] which leaves the price of 1 Gold at 0.0199€. You can, however, get the Gold with a discount of up to 40% if you buy a higher amount of it [which means that with a 40% discount 1 Gold would be worth 0.01133€].
Coins are the normal curreny in Standoff 2 and can be earned by playing the game, watching ads and converting Gold into Coins. Coins can only be used to buy normal skin boxes.
The convertion rate from Gold to Coins is 10 Gold for 100 Coins which makes 1 coin worth 0.00199€ [or 0.001133€ if you take the 40% discount]. You can't convert coins into Gold.