If you can only get drilling layer in gerber file, please try to convert it to excellon format.
Here is the guideline to convert drilling layer from gerber format to excellon format.
1. Open CAM350, File-Import-AutoImport
2. Find the Gerber file folder, then click finish button.
These gerber are generated by PCBE.
3. Tool- NC Editor
4. Click Gerber to Drill
5. hole.grb is the drilling layer in gerber, so we choose Source layer to hole.grb. Then click OK.
6. Now we get a new drilling layer(Layer_9).
7. File-Export-Drill Data
8. Choose the drilling layer as export. Change the file name to hole.drl that we can understand its meaning better.
9. Now we get correct drilling layer format.