I believe the original steel was 18 gauge, but when you are buying reproduction panels that are not original GM panels you can get some variances. Their are only a couple companies that make panels and then for a particular panel you may only have one to choose from. Companies save money or can sell the panels for cheaper costs by going to 20 gauge over 18 gauge and when your dealing with full frame steel cars you will probably never notice the difference.
Ya know, as a never-trained newbie welder, it took me several attempts at welding with inferior metal patches before a friend clued me in to how the metal quality between 2 pieces - despite being similar gauge - would affect how you weld 2 pieces together. I couldn't understand why I kept blowing through on the patch.
Sigh. School of Hard Knocks..........