Yeah you can use it for 3D printing, if I remember correctly each machine has their own spec for it, its almost like "assembly" for 3D printers.
Correct. It's how machines can implement new functionality (like multicolor, which entails additional steps like withdrawing the current filament, purging the nozzle of the current color, etc)
I think of it more as like the modem AT command set, but for the simpler side of industrial robotics.
Yeah, I remember writing gcode on paper in school to control a wire EDM machine
The test gcode in the repository adds almost 5mb to the crate size you might want to consider excluding[1] them