Recently flew through PVG and had access to the Air China lounge as I was flying SQ business - I only had Air China business class lounge access, not sure if GS gets you into the First Class lounge but I don't think you'd have access with Polaris alone.
IMO while the seating and space was nice, the food, etc. was pretty horrible. It felt more like a United Club (albeit more food options - just nothing that tasted palatable) than a business class lounge and definitely not in the same universe as the Polaris lounges. I had ~2 hours there which was about an hour and a half too much...