GAS is distributed by holding NEO in your wallet. Do note, many exchanges do not award GAS, so the wallet must be one where you hold the private (WIF) key.
GAS is used to fuel transactions on the NEO blockchain. As an example, deploying a smart contract will require GAS.
The rate at which GAS is awarded depends on the estimated block generation rate based on the amount of blocks generated the last 24 hours (per day).