Me likes what me likes!
I'm a beginner, I assume he is as well otherwise he wouldn't be asking, and 16ga certainly allows the most room for erroroneous strokes. If you can't get it perfectly smooth(like me), 16ga has some thickness to allow a bit of grinding.
I'm also used to workin on my mustang which has pretty darn thick panels. Not 16ga, dunno what i was thinkin. probably 18. But I can't find anything between 16 and 20 ga. 16, 20, and 22 is what I can get locally.
Listen to GTmike, I don't do much bodywork. Engine fabrication here...
What car are you doing this to? Picture? What is a 'tag'? Liscence plate?