The Chinese pair had already beaten the world record earlier. And they had put on a dominant display in the final of the women's team sprint track cycling event to beat their German rivals, Miriam Welte and Kristina Vogel, in the London Velodrome.
But their joy suddenly turned to bitterness as the Velodrome speaker announced that they were disqualified for a lane change in the final.
The Chinese rushed to the officials table, next to the paddocks in the centre of the track, and shook their heads in disbelief while looking at the amended results on a computer screen.
Vogel and Welte checked a computer screen several times and erupted in laughter and happiness as they were proclaimed winners in a time of 32.798 seconds.
"We really could not believe it when we saw it on the screen that we were Olympic champions," Vogel said. "It's amazing. It's weird and amazing."
Guo, a former world champion in the track cycling event of keirin, and Gong ended with the silver medal and managed a smile on the podium, but their eyes were still red with tears as the Germans waved to the crowd.
"This is a competition, so I don't want to say anything right now," Gong said.
On a frantic first day of competition at the Velodrome, known as the Pringle because of its potato chip shape, British riders Jess Varnish and Victoria Pendleton were also disqualified for making an early change in the first round.
During the two-lap race, the leadoff rider is required to give way to the second rider at a certain point on the track. The decision was met with boos from the crowd of 6,000 fans.
The decision dashed the medal hopes of the home favourites, who broke the world record during qualifying only for the Chinese to twice lower the mark, once in qualifying and once in the first round.
"It wasn't the way we wanted to win because the Chinese girls had been fantastic," Welte said. "The GB girls had been disqualified for the overtake, and I told Kristina: 'It's a chance, maybe we can get a gold medal.'"
Pendleton and Varnish had posted the second best time of the first round and would have faced the Chinese for the gold medal had it not been for their rule violation.
"It was an illegal change," Pendleton said. "I came through in the change zone about a meter too early. We are talking about one hundredth of a second of a mistake there."
It was not the first time British riders were disqualified at a major tournament. Sir Chris Hoy, Jason Kenny and Philip Hindes, who won the men's competition on Thursday, were disqualified at the worlds earlier this year for the same reason.
"Jess moved up a fraction too early, and I just saw the door and went for it, because that's my cue to try to squeeze underneath her as quickly as possible," Pendleton said.
"It's one of those things that happens. It's quicker than a blink of an eye. You have to stick by the rules. The rules are there to make it a fair sport. Unfortunately we fell on the wrong side of that today."
In the match for the bronze medal, Australia defeated Ukraine in 32.727 seconds. Anna Meares became the most decorated woman's athlete in track cycling by claiming a fourth Olympic medal.
"Drama all night," Meares said. "I can only imagine how devastating that was for the British. The worst thing you can do is add emotion to a bunch of sprinters."
Meares, teamed with Kaarle McCulloch, is expected to compete in the keirin and the sprint, where she will be facing archrival Pendleton.
The British rider will retire after the London Games and is bidding to add medals to the Olympic sprint title she won in Beijing.
Pendleton won the sprint at the worlds after defeating archrival Anna Meares of Australia in the semifinals.
AP Sports Writer Dave Skretta in London contributed.