Gas is a “fee” for every smart contract that you wish to run on Ethereum. To activate a smart contract, you need to ask Ethereum miners in the whole network to individually perform the calculations on your smart contract and gas is a way to pay the miners for their services.
Gas is not an actual token that you can “own” even if it's a unit that can be measured. Gas exists only on the Ethereum Virtual Machine. If the smart contract is complex and requires a number and type of computational steps, memory used for storage, and many other requirements, then more Gas is necessary to execute it. So, if you need to pay Gas, you will be charged with the number of Ethers.
Why do we use Gas and not Ether directly?
Similar to Bitcoins, Ether has a market price that is very volatile, and thus the price changes dramatically. And because of this, it is beneficial if there is a separation of the price of the Ether vs Gas so that the cost of operations of miners will not change even if the market price of Ether changes frequently.
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