Use In Game Coin to easily implement cryptocurrency in your game!
Though it's not yet audited for usage at commercial projects, this service is already perfectly suitable for test projects or even hobby projects!
For a sample game that uses this API, Play Cactus Land, or check the Unity Project.
With the withdrawn game coins, you can trade with others, potentially in Serum or any Exchange or Automated Market Maker.
Or eventually, even send them across different games!