Is 20 gauge OK for general body patches?

Down here paint was crap in the 60's and the 70's even the early 80's, but the 70's cars rusted out faster than the 60's, especially Australian cars, some of that was structurally poor design, some was poor metal, all was poor paint. My first car was a Chrysler assembled Mitsubishi, it had a couple of small rust spots when only a few years old, and that was considered very good for a Japanese car. Hondas and Toyotas started to blister instantly. By comparison my 75 Duster seems painted to a high standard to what we had down here, certainly better than my 69 dart, but the dart had galvanised rocker panels which really helped.