I recently designed a board and got them produced. I had specified plated holes at some locations. But the PCBs came without them (unplated). I decided to check the Gerber files to see if it was plated info. The design sure has plated holes. How can I check in Gerbers the plating information ?
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).
Hole plating information is not normally a part of the actual Gerber data information. Instead it is included in a separate file called a Drill file. A common format for this file is called the Excellon data file format.
Some gerber data viewers can interpret the drill data files for you.
It is also common practice to also produce a separate CAD file called a Fabrication Drawing with your project that includes a table of drill hole sizes and summary showing also which holes are plated and non plated. It is normal to show all non-plated holes on the Fabrication Drawing on a dimensioned outline of the board.