The gold coins are currency among the Assassins. It's not made clear how exactly one earns them or why they don't just use real money. But it can be speculated that you earn a certain amount of coins for certain missions. These coins can be exchanged for services within the assassin industry, such as removing bodies and cleaning up any forensic evidence, purchasing weapons and armor, information, and so on. It can also be assumed they use this form of "in-house" currency to avoid any real money being marked or traced to anyone in the organization. Having a private form of currency also makes it more secure against forgery or anyone outside the organization using it themselves.