OK. Your posted sounded like you were seeking user documentation and quite frankly there is no one compendium.
You’re right in that G-code documentation is very fragmented. Part of the problem is that G-Code was borrowed from 30 year old CNC machine technology back when RepRap printers first came on the seen. It has ben extended quite a but since then into supporting 3D printing. Essentially CNC is removal of material with a cutting head and as you know, 3D printing is an additive technology.
Part of the challenge you will face is that there is no one beginners guide per-se . Although there are may “reference” guides. So unfortunately, no one-stop shop to pick up the tech easily. What I would recommend is simply going to YouTube and searching for 3D Printing Tutorials. That’s where I learned most of my tips.
Check out the two reference links in this post. They are probably a great place to start because at the very least, the reference are for G-Code used in 3D Printing. Also, I know that Wikipedia has a great reference page that has loads more links. But at the end of the day, you will find yourself going down many rabbit holes as some resources will be too fundamental and others too deep or narrow for what your looking for.