It depends on the year. Older cars use thicker steel. For example, the steel used on the 64 Karmann Ghia I used to own was 0.9mm. 70s cars use 0.8mm. Repro body panes are thinner still.
I think .8mm is close to 20 ga.
If you are making small panels, try to use genuine VW steel, it is the right thickness. Also, there is less chance of rejection.