excellon
There are 3 repositories under excellon topic.
XRay3D/GERBER_X3
Program for preparing G-code for milling of printed circuit boards on CNC. You can use it as a converter to a PDF file.
snegovick/bcam
CAM system for hobbyists and shapeokers
vdm-dev/StepCAM
Sprint-Layout PCB Export to G-code converter