The Toronto native attended Notre Dame College in Wilcox, Saskatchewan and was a standout defenseman for two seasons with the WHL's Regina Pats where he was voted the league's Top Defenceman and First Team All-Star. He was chosen 24th overall by his hometown Toronto Maple Leafs in the 1982 Entry Draft and then returned to junior where he scored 86 points in 63 games.
Leeman also represented Canada at the World Junior Championships on two occasions, in Leningrad and Sweden respectively.
A speedy player with soft hands, Gary Leeman played nearly 700 NHL games for five different clubs. He was best known as a gritty scoring machine on the Toronto Maple Leafs with a 50-goal season to his credit.
By the 1985-86 season Leeman was part of the "Hound Line" along with Wendel Clark and beginning in 1986-87, Leeman was a top goal scorer for the Leafs with four-straight 20-goal seasons. In 1988-89, he scored 32 goals and was picked to play in the NHL All-Star Game despite missing 19 games due to injury.
In 1989-90, he formed the potent "GEM Line" with Ed Olczyk and Mark Osborne and became the second Toronto player after Rick Vaive to register a 50-goal season and became the first player drafted as a defenseman, converted to a forward to reach the 50-goal plateau. He dropped to 17 goals the next season when his effectiveness was limited by a shoulder separation.
After nearly nine seasons in Toronto, Leeman was the key player sent to Calgary in the blockbuster trade that brought Doug Gilmour to Toronto. To date, the 10 player deal is the largest in NHL history.
Leeman's time in Calgary would be short-lived. Seeing his ice-time dwindle, Gary asked for a trade. His request was granted and the sniper was shipped to Montreal in exchange for veteran forward Brian Skrudland. The move would prove to be fortuitous for Leeman as he and the Canadiens would go on to capture the Stanley Cup later that season.
Leeman would finish his NHL career with stops in Vancouver and St. Louis and would play three additional seasons in Europe before retiring in 1999.