So can gerbils gnaw through metal cage bars at all?
If they have enough time to gnaw through cage bars they will try. However it will take them a long time to do so.
They have very sharp teeth and so constant and consistent gnawing at the same bar will eventually ware it down.
When they gnaw on bars there is a real risk of them developing problems on their mouths such as broken teeth or even mouth sores.
This can give them real issues with eating.
This is why metal cages are not the best option for them.