Thread Gauge for Unplated Male Threading

Your final application is plated - so it must answer to the gauge after plating.

However the plating thickness , under normal cases are so low, that unless you thread originally to MMC it will answer to gauge either way (before and after).

But acceptance test is after plating only.

If the male shaft is unplated, it doesn't matter when you use the gage. It's the nut that's plated; that's what needs checked. But, seriously, should you be plating the nut if you have assembly issues? If you try measuring the thickness (that's your real question) down in those interior threads, you'll discover the meaning of a "fool's errand". Consider another material.

Thread gages typically measure pitch diameter, with thread form a secondary consideration. Typically, the gaging major and minor diameters never contact the inspected major or minor diameters unless the form is way off. Pre-plating thicknesses can be accommodated by special-order gages, but the previous posters were correct in that final acceptance is after plating.