Gagnon, of Lac-Etchemin, Que., finished .50 seconds behind American winner Mikaela Shiffrin. France’s Nastasia Noens was second, .45 seconds behind Shiffrin.
Shiffrin earned the victory in her first race after missing two months due to injury.
Crossing the line smiling after her second run, Shiffrin turned to look at the timings board then put her head in her hands at the realization she had won.
“I came in blind a little bit,” she said. “I knew my training skiing was some of the best I was doing but I just wasn’t sure. It was kind of close your eyes and go, see what happens at the end of the run.
“I didn’t know what to expect from the rest of the girls. I was just watching on television and wasn’t sure of the pace. I could see it but it’s different from being there.”
It was Shiffrin’s first race since tearing a ligament in her right knee in December. The Olympic and world slalom champion started skiing again only two weeks ago.
Shiffrin won two slaloms in Aspen, Colorado, and both by big margins, before the training mishap in Sweden. She has won her last six slaloms stretching back to last season.
It was Shiffrin’s 18th win on the circuit, moving her level with Tamara McKinney for the second-most wins by an American woman. She turns 21 next month.
Three of the top 10 after the first run did not finish: Michelle Gisin, Petra Vlhova and Erin Mielzynski. Lila Lapanja of the United States also skied out.
World Cup slalom leader Frida Hansdotter finished sixth, 5.89 behind Shiffrin. The Swede has 545 points, 99 more than Veronika Velez-Zuzulova of Slovakia, who was 10th.
Shiffrin, who has won the past three titles, is 245 points behind Hansdotter. There are only two slalom races left along with a parallel event this month in Stockholm that counts in the slalom standings.
The Associated Press