Well, I know that it has fixed gas limits for a particular operation but the gas price still varies. So why we need all these complications? Why not use ETH for transactions? Gas price and gas limit cost some Ethereum anyway, right? They are not separate currencies?
ETH is the ticker symbol for Ether, Ethereum's currency for compute processing. Ether has been nicknamed "gas" because in science ether is a gas.
ETH and gas are the same thing.
The limits for an operation represent the processing limits for a transaction on their blockchain network. The price of ETH/ether/gas is a measure of supply and demand for that processing power.