GAS is a cryptocurrency by NEO that is generated with each new block on the network. GAS is to be used to pay for using the platform (right now a lot of things are free but they wont be going forward). Contracts, instructions, asset registrations and changes all require GAS. When GAS is used, it is not destroyed. Instead, it gets recycled once it is used to pay for services and redistributed to NEO holders. In turn, NEO holders then sell it to people to pay for transactions, over and over again.
NEO is a network where people can build decentralized applications and smart contracts on top of the NEO blockchain. The smart contracts live permanently on the blockchain and can be written in multiple coding languages which is the main advantage of NEO that Ethereum lacks. Many people do not understand that NEO represents shares on the blockchain while GAS is the actual coin on the blockchain that is used to fund ICOs and smart contracts. In addition, holders of NEO also get paid GAS on a daily basis which acts as a dividend.
Full article: https://cryptocolumn.com/neo-and-gas-coin/