Need correct gas cap that fits flush

Nope. The gasket is supposed to be just like it is. He’s got the wrong cap. He might be able to tighten those screws a bit, but that does not account for the cap being a 1/2” out.

My car is missing the gasket, it’s supposed to be there.

Yeah but on the inside of the quarter between the filler neck and the inside of the quarter.