There are "real" on-chain games where the code is in the smart contracts. You can play by interacting directly with the smart contracts.
IIRC EOSBet is one example.

Unfortunately, I don't know much about decentraland. But if the game is complicated (unlike dice), the chance is it's only partially on-chain. Furthermore, if it's on Ethereum, no way the fees and speed will be adequate for on-chain games.