Gasser, 30, stomped three clean tricks in the contest, including a cab double cork 1260 on her final attempt, to claim the win. Sadowski-Synnott, who fell on her own 1260 attempt, took silver, and Japan's Kokomo Murase earned bronze.
In big air, riders are given three runs to land a trick, and their best two scores get added together. Riders, however, are required land two distinctly different tricks in order for those scores to count.
Most of the contenders appeared to come into the contest with a similar strategy: put down a pair of clean runs — typically a frontside double cork 1080 and a backside double cork 1080 — to start the contest, then go for broke on the third and final attempt.
At the end of two runs, four riders — Sadowski-Synott, Gasser, Murase and Reira Iwabuchi — had successfully put down 1080s in both directions and were separated at the top of the leaderboard by just 11 points, setting up a dramatic sequence to end the contest.
The action started with Iwabuchi going for a frontside triple underflip but being unable to land it. Murase also came up short on her final trick, which brought the battle down to Gasser and Sadowski-Synnott.
That's when Gasser dropped in and nailed the cab double cork 1260, earning her the highest single-trick score of the contest and putting her into the lead above Sadowski-Synnott.
Sadowski-Synnott, the 20-year-old from New Zealand, tried to respond with a backside double cork 1260 but was unable land the trick, clinching the win for Gasser.
"The level has been so high, all the girls were riding so well and I just wanted to show my tricks today," Gasser said afterward. "It was surprising that I'm on top of the podium again."
Throughout her storied career, Gasser has been the first woman to land a number of tricks, including the cab double cork 1260. She was the only rider to land any variation of a 1260 cleanly in this contest.
Before the contest began, there had been talk that Gasser might attempt a triple — something she has landed in training but no one has landed in a competition yet. Iwabuchi ended up being the only rider to try one, and although she fell, it earned the respect of her fellow competitors that were watching from the top of the course.
"It was amazing," Gasser said of that moment. "Everyone on top freaked out. That was a boss move and I wish she'd landed it even though that might have meant me being further down the ranking."
The significance of the moment was not lost on Sadowski-Synnott either.
“I was just thinking I am so stoked to see this live because it is history," she said of Iwabuchi's triple attempt. "It is a trick that has never been done and that is super special. I hope she gets it soon.”
With the win, Gasser has become the third snowboarder to defend their Olympic gold medal at these Games, joining Chloe Kim (women's halfpipe) and Ester Ledecka (women's parallel giant slalom) in accomplishing that feat. But according to Gasser, this year's medal filled her with more emotion than the first because she wasn't such a heavy favorite this time.
"It is a big difference to me. In PyeongChang, I felt that I was the big favorite as I'd won so many big airs leading up to it. I had tricks no one else had," Gasser said. "This time I was feeling it is going to be hard to get on the podium, but I was going to show my best. This one feels way more unexpected."
Meanwhile, silver medalist Sadowski-Synnott has now completed the trifecta of Olympic medals after winning big air bronze in 2018 and slopestyle gold earlier this month.
Results: Women's Snowboard Big Air
🥇 Anna Gasser (AUT)
🥈 Zoi Sadowski-Synnott (NZL)
🥉 Kokomo Murase (JPN)