The short answer is that France turned Guangzhouwan back to China in 1946 as part of a deal to get Chinese troops out of northern Indochina. 

 The longer answer is that Guangzhouwan was never a very successful concession. It was originally supposed to be a rival to Hong Kong, but it never panned out even as a rival to Macao. This was not that unusual.The foreign powers often took concessions in China with grand plans and then they never worked out. When Chinese revolutionaries seized the British Concession in Hankou/Hankow in 1927 the Brits gave it up in part because there was really no point in keeping it. 

Guangzhouwan was actually a minor embarrassment for France, since its main business was smuggling, especially opium smuggling. This made it a minor annoyance for the Chiang Kai-shek government before the 1937 Japanese invasion. After the invasion it was semi-useful as “China’s Casablanca”, a place to slip supplies and people in and out of Free China through a “neutral”port until it was occupied by the Japanese in 1943.

Chinese troops moved into Tonkin (northern Vietnam) in August 1945, and the French were very eager to have them out. They were also eager to have the Chinese stay out of Indochina in general (Chiang was trying to extend China’s influence into S.E. Asia.) The1946 agreement dealt with things like the status of rights along the Haiphong-Kunming railway and the return of the French Concession in Shanghai. This had in fact been returned by the Vichy government to the Wang Jingwei puppet regime,neither of which were recognized by China or France, but of course the French did not want to open that can of worms. The return of any concession made Chiang Kai-shek look good, and so Guangzhouwan just got wrapped up with everything else. 

Sources

  Steven Pieragastini State and Smuggling in Modern China: The Case of Guangzhouwan/Zhanjiang Cross-Currents: East Asian History and Culture Review (2017) 15: 108-139 

Philip Thai China’s War on Smuggling: Law, Economic Life,and the Making of the Modern State, 1842-1965. Columbia University. New York,NY: Columbia University Press, 2018. 

Peter Worthing  Occupation and Revolution: China and the Vietnamese August Revolution of 1945. Berkeley:Institute of East Asian Studies, University of California, Berkeley, 2001