BTC is known as the first cryptocurrency created. It allows for the decentralized public ledger to be used for peer-to-peer exchange. Payments can be settled faster and have lower transaction fees than traditional currency exchange. The peer-to-peer transactions allow for anyone to be involved, not just one single institution or entity.
Ethereum is an open source project. This means anyone is allowed to deploy decentralized applications. It is used in finance, databases, art, games, and many other projects. Other than Bitcoin, there is no bigger cryptocurrency by market cap. The cryptocurrency generated by Ethereum is called Ether (ETH).
GALA is the native cryptocurrency used in the Gala Games Ecosystem. Players can be rewarded for games and GALA can be used to buy items in the Gala Games Ecosystem or traded with other players.