G codes and header when launching the program

Where is the list of G codes and how to use them?
How do I generate the program head so that I always have the same one?

In Easel, you can “download” the gcode from your particular file. . That is a text file that opens up in a word processor. You can add to the gcode by typing something like this, ( text within parenthethes). That doesn’t impact the movement.

Also in Easel, your files are not removed unless you delete them. If this doesn’t answer your question, perhaps you can elaborate.

Grbl V1.1 Quick Reference – SainSmart.com

grbl/doc/markdown/commands.md at master · gnea/grbl · GitHub

What I would like is for the program to automatically come out with the header in G code, for example:
G90 G80 G49 G40
G28Z0
G…
G…