Gas in Ethereum is based on calculation complexity, and there is no direct equivalent of this in MultiChain, since most MultiChain transactions do not perform these sorts of general purpose calculations.
You can however define fees in your blockchain based on transaction size, and also size of references off-chain data if you're using off-chain stream items. Please see the 'Native blockchain currency' parameters here:
https://www.multichain.com/developers/blockchain-parameters/