Are you talking about the gauge of the steel that was used in the inner fenders (or fender aprons) or in the frame rails or in the firewall or the floor toe-pans or in the radiator support or in the battery box?
Why not just buy some reproduction parts and if you want to piece-in parts from them you will have the correct shaped or contoured part as well as what should be the correct gauge of steel in the part. I believe most of the reproduction steel body panel parts are same gauge as the originals, yes? I'm not sure though.
+1 on buying the parts you need if at all possible. But if you're just fixing a small, flat hole or damage, I think it was 16 gauge. Repro stuff is more like 18 guage, but once you weld it in place, it's tough to tell.