Warsaw (Poland)-Guangzhou-HK-Warsaw

The first detail is that 72 hours were replaced with more generous 144 hours in entire Guangdong province (where Guangzhou is located) years ago.

You have mentioned your route and it would be qualified for 144 hour visa free transit scheme, but as far as I know, there is no direct flight connection between Warsaw and Guangzhou. You have to be very precise in this segment because everything depends on that. Just technically, I will tell you, if only there was a direct flight from Warsaw to Guangzhou then your entire itinerary would be qualified for this visa free transit scheme.

Beside the ferry, you can also fly from Guangzhou to Hong Kong. If you plan to take a ferry you can do it by taking a direct one to Hong Kong. You can use several departure ports like Pazhou (located next to Canton Fair Complex, so quite close to the center), Nansha (south from Guangzhou and reachable by metro), Shekou in Shenzhen and some other. Please, have in mind that you will have to buy your ferry ticket through CKS - Chu Kong Passenger Transport Company in advance and to print this ticket out. You will have to have it in hard copy, ready to present when needed. Also, you will have to take exactly that ferry you have bought your tickets for.

Now, please, check your itinerary carefully and see what will be the immediate point/airport before Guangzhou.

Thank you very much for such a detailed reply!
Based on it we change our plans - we will go to Beijing spend there 72 hours and then go to HK. The flight is from Warsaw through Istambul to Beging, then to HK (and back to Warsaw after few days). Would it be ok to get the 72h free pass in Beijing based on this itinerary?
Looking forward to your kind reply.

Yes, there is no problem of your itinerary.

Grzegorz, please have in mind that this is 144 and not 72 hour visa free transit. It used to be 72 hours, but it was expanded to 144 hours years ago even for Beijing transit area.