in a nutshell :

gas is the cost unit

gas price is a single gas unit's price

and

Fee= gas*gas price

why using gas instead wei? because we need a fixed value (unit) for expressing the operations cost. then this initial cost is "translated" in wei/ether which may vary according to the market.

"y operation"= x gas => x gas * market gas price= your fee.

in a nutshell :

gas is the cost unit

gas price is a single gas unit's price

and

Fee= gas*gas price

why using gas instead wei?

because we need a fixed value (unit) for expressing the operations cost. then this initial cost is "translated" in wei/ether which may vary according to the market.

"y operation"= x gas => x gas * market gas price= your fee (Z).

so "y operation" cost will be always x gas, today as in next year, but z will change as the gas price will change.