Olympic gold medallist Steve Guerdat of Switzerland took second place riding Nasa and France's Penelope Leprevost finished third with Nayana.
Eric Lamaze of Schomberg, Ont., placed eighth with Powerplay.
Devos lost his stirrup after a triple combination, but he and Candy managed the only clean round in a time of 70.99 seconds.
It was the 27-year-old's first time competing at Spruce Meadows.
"I was thinking you would have to come here two or three times before you can perform in a really good way. You have to know and train," Devos said. "For me, it's a very big surprise that I won here. Now I start to realize what I won and it is just great.
"It's a dream come true. To win here is great, but to win the grand prix is unbelievable."