Gerard scored 87.16 on his final run to jump ahead of McMorris, who topped the standings with a score of 85.20 following the first two runs.
Gerard believes taking a risk in the final run was the difference between himself and Parrot. "It could have been a lot worse so I'm just really glad to be snowboarding and having fun again".
When his time came, Gerard nailed it. "So I might as well try and do good".
Gerard the youngest person to ever win an Olympic gold in snowboarding.
Canada's Mark McMorris left everything out on the course during Sunday's slopestyle final, securing a second Olympic bronze medal and completing a heroic comeback from injury.
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Lightning and light rain had first intervened during the Indian innings, and it came again early during the South African innings. He tentatively fended at an incoming delivery and lobbed the ball back to Rabada who took a sharp catch in his follow-through.
Gerard raised the bar with his backside triple cork 1440 during his final run to take the lead off McMorris and the Canadian couldn't respond, falling on the final kicker. He also managed to edge Parrot, who turned in his best score of the day on his third and final run. The final rounds will go Tuesday morning.
The 18-year-old, who was the first snowboarder to take to the slope, scored 70.85 on his first run but it was only enough to finish ninth and miss out on qualification.
"I think it doesn't really matter (what medal you get) as long as you are out here riding and boarding as good as you can", enthused McMorris.
Each athlete has two attempts to score the highest score possible, with each competitor's best run counting as their qualification run. Sochi Games silver medallist Staale Sandbech was fourth, with teenage sensation Marcus Kleveland a disappointing sixth.