Neither Plaza Premium (LX) or CNAC are *G lounges, they are contract lounges, which means you have to fly on the specific airlines that issue a lounge invite to gain access, same as VS Clubhouse (BR). For example, a UA*G flying UA or SQ*G flying SQ or AC*G flying AC would NEVER get access to any of those lounges !

Btw, Air China has no lounge in HKG, Air China use that CNAC lounge in HKG, which OZ also use it (base on it is closest to the OZ gates). CNAC lounge is a lounge mainly used by all those Chinese carriers like CA, CZ, MU. (CNAC is basically equivalent to CAAC, the former parent of CA, CZ, MU !)