How do you know what resolution a gerber file has?
Gerber files don't have a "resolution" as such. They are vector representations - lines, circles, fills. So they can be rendered at whatever resolution is useful at the time - scaled up or down onscreen without pixelating.

For manufacture, the Gerbers have a header which explains the units and precision of all of the points. This ensures that they are manufactured at the correct size. The manufacturer then makes their masks at whatever resolution they require by scaling the vector representation based on the units.