There’s several specifications for the GCode language. So here’s the links to two official GCode specifications.
NIST spec -
nistir6556.pdf (The same document can also be found at get_pdf.cfm (nist.gov))
If they move it, you can search NIST ‘publications’ for RS274NGC Search Results | NIST RS274NGC
ISO spec -
This is more up to date - unfortunately the ISO wants to charge CHF124 for it (that’s currently USD$141)
ISO 6983-1:2009 - Automation systems and integration — Numerical control of machines — Program format and definitions of address words
Search ISO for 6983