GasLimit is measured in units of gas and set by the message sender. It imposes a hard limit on the amount of gas (i.e., number of units of gas) that a message’s execution should be allowed to consume on chain.
GasPremium is the price per unit of gas (measured in attoFIL/gas) that the message sender is willing to pay (on top of the BaseFee) to “tip” the miner that will include this message in a block.
GasFeeCap is the maximum price that the message sender is willing to pay per unit of gas (measured in attoFIL/gas unit). Together with the GasLimit, the GasFeeCap is setting the maximum amount of FIL that a sender will pay for a message: a sender is guaranteed that a message will never cost them more than GasLimit * GasFeeCap attoFIL (not including any Premium that the message includes for its recipient).