So far in my progress (only reached Lima), from certain missions in Campaign. There's one mission that rewards you around 25 gold coins in each city.

Gold coins can also be won in raids and PvP. Also, you can switch between the Zombies and Survivors and share gold coins gathered between the two.

Oh. More stuff! Okay, you can sometimes get coins in Rio de Janeiro and New Orleans as rewards.

Yeah, and once you use your gold sides on one side, they're not on the other side as well.

In the later levels you can get 2 missions which reward 25 coins. For example Jakarta and Bangkok have the opportunity to get 50+ coins.

- Tomowato / Tomozombo Leader of Djembi / Djzombi