Gerber verification for plated holes
Drilling is usually defined by Excellon files, not Gerbers. But either way, the file itself carries no information about plating. There may be textual notes generated by the CAD software, but no machine-readable code.

Whether the holes will be plated or not depends on the board manufacturer and most of them require two separate files with specific filenames (for example pth.exc for plated and npth.exc for unplated).