I’m afraid those are all commercial machines — how does the “Generic” one work?
One of the smaller mill manufacturers, Tormach? has shifted to using LinuxCNC — this page indicates that one can work up one’s own post-processor: http://traditional.surfcam.com/mpost
Documentation on the variant of G-Code which Grbl accepts is here: https://www.shapeoko.com/wiki/index.php/G-Code and on the Grbl githup page.