What youth hockey league did Wayne Gretzky play in before the NHL?

Before reaching the NHL, Wayne Gretzky played in the Ontario Hockey League. He joined the Sault Ste. Marie Greyhounds for the 1977–78 season, when he was 16 years old. The OHL is one of Canada's three major junior hockey leagues and is part of the Canadian Hockey League.

Gretzky immediately stood out in Sault Ste. Marie. As an OHL rookie, he scored 70 goals, 112 assists, and 182 points. His 70 goals set an OHL record for a 16-year-old, while his 182 points ranked among the league's best totals. He won both the Emms Family Award as rookie of the year and the William Hanley Trophy as the league's most gentlemanly player.

The question refers to his major junior path, not every league he played in. Gretzky had already appeared in the World Hockey Association with the Indianapolis Racers and Edmonton Oilers before becoming NHL-eligible, but the Ontario Hockey League is the canonical answer for his youth-hockey league immediately before his NHL career.