The Guangzhou FIR is just to the north of Hong Kong and extends a fair way east and west. Any traffic arriving or departing Hong Kong from/to the north-west, north, or north-east (or anything in between!) will fly through Guangzhou airspace. That covers pretty much all of the traffic coming from or departing to mainland China.
If you have a look at Flight Radar 24 as Hugo suggested, you will see that the rest of the traffic arriving or departing Hong Kong does not fly through Guangzhou's airspace.