Aswell as there being two types of silver, there are two types of gold.
The first, unbound is used to buy things in the (bound) gold shop. It is gained from leveling up and few other ways that I am not sure of.
The second kind of gold is unbound gold. It is bought with real money and can be used to buy and do many things. You can play the lottery in Heaven, shop in the shop, and buy things in the market.