The 17-year-old landed an 87.16 point final effort in the slopestyle to snatch victory from Canadians Max Parrot, who took silver, and Mark McMorris, who picked up a second bronze after Sochi four years ago.
"I was really happy when I saw I came in as first," said Gerard.
"I don't know, silver (Parrot) had a pretty good run, I was questioning it.
"But you know, judges have their own minds and I'm super-psyched!"
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"It feels incredible. I'm just really happy that I got to land a run and I'm just really excited right now," said the Colorado native, who came fourth in the X-Games last month.
Gerard tried the sport as a toddler then took it up with a passion aged eight when his family moved closer to the snow.
He built his own snowboarding training ground with his brother at the back of his family home as his ambitions grew.
"It was awesome. I just told myself that I want to land a run and I was a little bummed on my first two runs because I fell a couple of times," he said.
"I'm just so happy that it all worked out."
Canada's Parrot, who won the X-Games Big Air this year in Aspen, Colorado, also had a disappointing first two runs but hoisted himself into silver medal position on the last attempt.