1st of all there isn’t a specific fingering key position on the sax to play an altissimo G, there’s several different fingering positions and they don’t work when you blow them.
2nd to play any altissimo note, there are several different finger key positions to play any altissimo note, but they don’t work.
3rd when you play any altissimo note, you are actually playing what is called an overtone.
4th you can play lots of different overtones on any sax key fingering position.
So start by learning how to play overtones first, once you can master how to blow overtones, then you’l realise that altissimo G is just another overtone. Could take several weeks, could take several months, but it is doable.
check out johnny’s altissimo course – that’s how i did it in a few months.