Gold is an in-game currency in Civilization used to purchase units and buildings (and enemy units, with the diplomat). It is also used to maintain buildings- if expenses exceed revenue and there is no gold stored up, buildings will no longer function.
Gold can be produced in a variety of ways:
- As an output of trade produced by cities (gold revenue can further be increased by building markets and banks);
- Creating a trade route with a caravan;
- selling buildings in cities;
- and demanding tribute from less powerful civilizations (when not under republic or democracy) after meeting with another civilization's leader.
You can view your gold revenue in the 'Commerce' tab under Advisors.