GAS is the secondary token on NEO ecosystem - it is used to pay for transaction fees for more complex transactions. It is also used to deploy and run smart contracts on the NEO blockchain. Unlike NEO, GAS is divisible, and does not need to be a whole number.
Owning and holding NEO in your NEO wallet will generate GAS over time (passive income). Each block (about 15 seconds), NEO holders receive a small amount of GAS for each NEO held in their wallet.
More information here