While rain fell in Queenstown, Coronet Peak was hit by rain, snow and wind, but 66 athletes braved the conditions to tackle the 31-gate course from the Hurdle to the Eighth Basin.
Gagnon (21), who holds a world ranking of 42 in giant slalom, produced the fastest first run, 52.42sec, and the third-fastest second run to take the gold medal and the title in a combined time of 1min 43.75sec.
Daniel Agnieszka-Gasiencia (21), of Poland, ranked in the top 65 in the world, was second in 1:44.37 after clocking the third-fastest run.
Super G silver medal winner Maria Pietilae-Holmner (23), of Sweden, claimed third with 1:44.49.
Gagnon said the first run was slow, owing to the soft snow conditions.
"I was like, `Come on, come on'.
"In the second run I sat on the ground on the third gate, but it was OK and I got through. It was really difficult conditions and I am very happy to win."
The first New Zealander home was Hamilton's Sarah Jarvis (23), who finished 20th overall with a time of 1:48.33, improving her world ranking.
Jarvis told the Otago Daily Times she was pleased with her result, given the challenging conditions and the accomplished field.
"It was pretty challenging conditions and we had to rise to that challenge . . . it was mostly raining, but there was a bit of snow . . . that made the snow really soft.
"I am happy - the top girls were really awesome and that shows in our penalty count being the lowest."
Jarvis said she was also heartened to be "reasonably close" to the top three place-getters.
Canada's Britt Jaynk had the fastest second run in the field with a blistering 50.67sec, which, after her first run (54.21sec) put her into fifth position.
Kristine Haugen, of Norway, won the award for fastest junior and finished seventh overall in 1:45.00.
