The surprise gold medal, though, caps the happiest time in her life, she said.
Since winning in the Beijing Games on her home range, Guo has married and had a baby daughter. That's why her results of late haven't been anything to rave about, she said. When Guo competed in the Chinese Olympic trials almost a year ago, she was still breastfeeding. A Chinese reporter asked her if being a mother had doubled her workload as an elite athlete.
"I don't think so," she said, "because that's what I would do anyway."
Two other Beijing teammates and gold medallists , Chen Ying and Du Li, have also married and had children since. They will also be defending their titles at the Royal Artillery Barracks this week.
It all adds up to Guo rating this second gold medal in London higher than the first.
"Beijing was my first Olympics, I didn't really think too much, I did an OK job, I hadn't set a goal," she said. "This time I had more desire. I had a goal and worked very hard and achieved my goal. That's why I'm more excited and this (gold) is more important to me."
Guo nearly missed the gold medal, but pulled off a stunning near-perfect last shot in the final to prevail. She scored 10.8 out of 10.9 with her 10th shot to rise above leader Celine Goberville of France, who blew the gold with an 8.8 and fell into a second-placed tie with Olena Kostevych of Ukraine.
As Guo celebrated with her teammates and coaches, Goberville won the one-shot shootoff 10.6 to 9.7 to win the silver medal in her first games. Kostevych, the 2004 Olympic champion, settled for bronze.
"I'm full of admiration for that performance," Goberville said of Guo. "Winning twice in a row is absolutely amazing."
Guo topped qualifying by one point from gold-medal favourite Kostevych and Goberville, and the trio shot well enough to stay in the medal positions ahead of the other five finalists in a dramatic final.
Guo began the final with a shocking 8.0 and fell to third. How did she recover? With a 10.8.
"You have 10 shots and I knew it was important to concentrate on every remaining shot," she said.
Kostevych lost the lead after her third shot, a 9.1, her worst in the final.
Dorothy Ludwig of Langely, B.C., finished 34th in qualifying to miss out on the final.
Guo was back on top in the fourth round, but Goberville hit the front in the sixth and stayed there until the last round.
It was the second shooting gold in two days for China, a day after Yi Siling secured the first gold medal awarded in the games by winning the women's 10-meter air rifle.
Goberville had won only once since 2009 but always been a contender. This was her 11th final in 14 events in the past four years, and the silver medal might keep her shooting. She halted her physio studies to work toward becoming a riding instructor, and bought her first pony last year.
"I can't believe I did something so good," she said.
Kostevych responded to a humbling 31st place as the defending champ in Beijing by becoming the most successful woman at 10 metres in the last four years, with twice as many wins as Guo and a win in the last World Cup last month. But she couldn't keep the momentum going for a second Olympic gold and a car promised by a sponsor.
"I am not the gold medal winner so I don't think anyone will be speaking about a car anymore," she said with a smile.
Afterwards, she criticized the Ukraine shooting federation for providing poor training conditions, and hoped they would do better. She also believed she could have done better, having felt the pressure as the favourite. She was keen to compete in Rio in 2016.
In the meantime, she was worried about her mother, who wasn't on site.
"Unfortunately," Kostevych said, "she must be somehow still on her way here."