It's a common misconception that the "G" name refers to the missile system. It doesn't, it refers only to the missile itself. It is also a generalisation used for convernience. The "Grumble" missile, used by both the SA-10 and the SA-20, can refer to either the 48N6 or the older 5V55 missiles.
The Russians have been spouting in open source about the S-400 system for over a decade. How far they've actually got with it bearing in mind their funding problems is another question...