Gas Fee

Gas is the fee you pay to the network to process a transaction. This is extra fee charged from sender. if sender is sending(1 ETH), gas would be (.002 ETH example). User has to pay 1.002 ETH

Ethereum Currency(ether, gwei, wei)

Any transaction on Ethereum network require a fee to be paid in Ether(ETH).
Wei is the smallest possible unit of Ether (ETH) on the Ethereum network. It works just like a "cent" for the U.S. Dollar

1 ether = 1,000,000,000,000,000,000 (or 10¹⁸) wei   // Smallest unit of ether

1 ether = 1,000,000,000 (or 10⁹) gwei   // Used for paying gas fees