If I fly in and out of Guangzhou with 72 hours , can I do a day trip to macau within that 72 hours

One possibility is to fly to HK on a, say 50-60 hour trip, and from HK take a day trip to Macu. (This idea is because there seemyobeno direct GZ-Macau flights)

Bram, according to the principle, you can only move around in Guangzhou when using the 72 hour free transit.
But you know, you can make a day trip to Macau, if you go there and return by flight, you can enjoy the 72 hour free transit again because Macau is regarded as a third region when speaking of the 72 hour policy.
Of course, if you travel by ferry, it’s okay, but in this way, you can only enjoy the 24 hour direct transit in Guangzhou when you come back.