Gagnon, from Lac-Etchemin, Que., all but secured her spot on Canada's team for the 2014 Sochi Olympic Winter Games with a 10th-place finish in last month's giant slalom opener in Austria and the versatile 24-year-old followed that up with two determined, attacking runs Saturday amid strong winds and varying light.
After two races she is sixth in the overall ladies' World Cup rankings.
"I'm stoked — just super happy that this race brought me fifth place and so, so happy for Elli. She was awesome today," said Gagnon, who had a two-run combined time of one minute 56.80 seconds. "It was a weird race — it was a bit of a free-for-all.
"I knew I wasn't far out after the first run. I skied just like I do in training — it was just like a normal run."
Terwiel, from Sun Peaks, B.C., found herself sitting seventh after an incredible first run and although she fell back a little on her second run an 11th-place finish represents her best-ever result in a World Cup race.
Britt Phelan of Mont-Tremblant, Que., Anna Goodman of Pointe Claire, Que., and Erin Mielzynski of Collingwood, Ont., did not qualify for the second run.
Terwiel, whose best World Cup results before Saturday's race were two 17th-place finishes — both in January, 2013 — took advantage of the wind dying down to put together a superb first run.
"When I came down at the end of the first run I was fourth and I did a little bit of an open-jawed spin-around in the finish. It was a little too good to be true," said Terwiel, 24, who had a two-run combined time of 1:57.30.
"A lot of things can go sideways so I tried to just focus on my skiing and the things I've been working on in training this summer. This is my best result ever and it's a top 12, which is one of the things I need for Olympic qualification. It's really exciting."
American phenom Mikaela Shiffrin made a big mistake at the end of her second run but still dominated to win with a two-run combined time of 1:55.07. Germany's Maria Hoefl-Riesch was second (1:56.13) and Slovenia's Tina Maze was third (1:56.68) — just twelve-hundredths ahead of Gagnon.
The 18-year-old Shiffrin led by half a second after the first run and raced seamlessly at the top of the hill in the second to extend her advantage, overcoming a mistake near the end to clock a combined time of 1 minute, 55.07 seconds.
"I'm really happy with how the day went and I'm also really excited because it looks like there are some pretty fast girls in the back of the pack," Shiffrin said. "I'm excited for the Olympics for sure but there are a couple of races between now and then, so hopefully I can just keep this going."
The basic qualification criteria for nomination to Canada's Olympic team in alpine skiing is two top-12 World Cup results, with at least one this season. A single top-five result this year also counts, so Gagnon effectively cemented her place at the 2014 Sochi Olympic Winter Games with Saturday's result.
She's the only Canadian to have pre-qualified in alpine skiing to date.