there’s… not a LOT to it.
I find that I normally just poke the wikipedia article (https://en.wikipedia.org/wiki/G-code ).
Gcode (as our machines understand it) is just a linear, top down set of commands.
normally there’s some basic one time setup stuff (and at the very end a M02 to end the program) and everything else is a set of G0 and G1 commands that are just XYZ movements.
for learning, I would recommend drawing a single line in Carbide Create, and then do a contour toolpath that just is on that line… save the gcode and open it in notepad (or rather, notepad++ since that’s much nicer)
with that, go through each line and look up what it does in the wikipedia article, and with that… you’ll know 90% of what there is to it ;_)