Playing in his second match for the Rio de Janeiro outfit, the 35-year-old combined with Rhuan Silveira before deftly sliding a shot past Cassio.
Elsewhere, Belarus' fifth seed Aryna Sabalenka lost to compatriot Victoria Azarenka 6-1, 6-3, ninth seed Johanna Konta of Britain was stunned by Romania's Sorana Cirstea 2-6, 7-6 (5), 6-4 and tenth seed Garbine Muguruza from Spain lost to Bulgaria's Tsvetana Pironkova 7-5, 6-3.
It added that Roma, Porto, Paris Saint-Germain, Ajax and Bayern Munich have also shown interest in the 23-year-old.
"Ensuring a safe return to international table tennis has been our utmost priority ever since the pandemic broke out. I am delighted that, with the fantastic support and hard work carried out by everyone at the ITTF over several months, we have managed to get events back up and running before the end of 2020," said the ITTF President Thomas Weikert.
"Now, I participate in eight to 10 races a year. I ran one triathlon and three marathons in a month last year," said Gu, who hopes to compete in the next Boston Marathon.
The under-23 striker Zhang proved his worth again in the 44th minute, as he tapped Jonathan Viera Ramos' cross home to add another goal for Guoan.
"I have been [wanting to leave] all year. I thought the team needed younger players and I thought my epoch at Barcelona had ended. It was a complicated year. I suffered a lot in training, in games and in the dressing room. Everything was very difficult and there came a moment when I decided to look for new objectives."
Ruichang took the first point with a straight win in the women's doubles, before Zhejiang fought back and took the victories in the men's singles and men's doubles. Then He Bingjiao overcame a one-set deficit to beat Han Qianxi in the women's singles, helping Ruichang to tie the game 2-2.
In addition to Hangzhou, co-host cities of Ningbo, Wenzhou, Jinhua, Shaoxing also staged water sports competitions and other Asiad-themed activities, joined by other cities in Zhejiang including Lishui and Jiaxing.
CTTA President Liu Guoliang called "it very significant that the ITTF has chosen China as the host to restart competitions after what has been an extremely challenging year for our sport."
Spain now move top of Group A4, one point ahead of second-placed Ukraine. Germany are third on two points after they lost 1-0 to Switzerland.