Gold is used mainly for reducing the cooldown period to zero. When you upgrade a building, levy or upgrade technology, it is finished instantly but you can't do the same action again until it's cooldown reaches zero. You can spend gold to instantly reduce the cooldown period to zero.
Gold can be bought using real money or gotten through taxing people.
Gold can be bought for US Dollars from the people at Clapalong.
Gold through TaxingEdit
You can upgrade the Assembly and get more Cottages which will get you more Drachmas when taxing. However, every once in a while, you can get 10 gold from taxing instead. A higher Mint level would give you a bigger chance of getting gold from taxing. The probability of getting gold from taxing is currently unknown, but it is understood to be extremely low.