That changes for each game you're looking to play. Generally, if you own some Ethereum, you're going to be okay. Most NFT marketplaces, even if they are built on other blockchains or primarily accept a different coin, usually also accept Ethereum. However, for Gala Games' marketplace, you're better off purchasing NFTs with Gala Coin. On Open Sea, you have the choice of Ethereum or Polygon. And on Rarible it changes for each game you're looking to join. But a generally good rule is to have some Ethereum.