From what I've found, most of the metal on the car is 18ga, with some of the floor firewall bits being 16. However, in alot of those sections, there seem to be multiple layers of it, all spotwelded together. I don't know if its best to replace like with like here, as I'm not sure of some of the processes used in unibody construction, but thats what Ive found at least about the thickness (where I've cut into it at least).
Update on this, I'm pretty sure the doorskins are 20-22 gauge. This could be important for anyone planning to shave the handles and weld them up.
