In a repeat of the result from the 2022 Beijing Games, Gremaud scored 86.96 in a stunning second run to edge out China’s Gu, whose best total in her three runs was 86.58.
Canada’s Megan Oldham took bronze with 76.46.
Gu seized control of the competition on her first run, punching the air after her 86.58 on the obstacle-filled course.
Gremaud, skiing last in the field, responded with 83.60 as the two favourites set a standard their rivals could not match.
But Gu made a mistake on the first rail of her second run, mustering just 23.00 points.
Gremaud, a day after her 26th birthday, then stepped up in spectacular style to grab the lead, heaping the pressure on Gu.
She could not respond, making a mistake at the top of the course to confirm Gremaud as champion.
The champion then had the luxury of a victory lap, cruising down the course with a Swiss flag around her neck.
The Swiss, whose Olympic career started with silver in the slopestyle at the 2018 Pyeongchang Games, won slopestyle gold four years later along with bronze in big air.
On the eve of the Games she said she had never lost her passion for skiing despite periods of grief and anxiety over the past few years.
She has faced multiple challenges since topping the podium in China, including the death of her aunt and a split from her manager.
US-born Gu, who switched allegiance to China in 2019, was one of the faces of the Beijing Games.
She became the first athlete to secure three freestyle skiing medals at a single Winter Olympics, winning half-pipe and big air gold and taking silver behind Gremaud in slopestyle.
The multi-talented skier, who models and studies at Stanford, was aiming for a clean sweep of three golds in all of her events at the Milan-Cortina Games.
In slopestyle, athletes navigate a course with a variety of obstacles, including rails and various jumps, and are judged on the breadth, originality, and quality of their stunts. (AFP)
