72 hrs transit visa. I am flying to hong kong with a stopover in Guangzhou.

To qualify for the 72 hour transit without visa (TWOV) you need to be arriving into china from a different country than you are departing to. Country A > Guangzhou > Hong Kong is allowed. Then you can fly HKG > Guangzhou > Country B. You are limited to one stop in China, Hong Kong and Macau are considered their own country for TWOV purposes.