Gareau competed for Canada in the 1984 Summer Olympics held in Los Angeles, United States in the 100 metres and the 4 x 100 metres relay, where she anchored Canada to the silver medal (as a 17-year-old) with her teammates Angela Bailey, Marita Payne and Angella Taylor-Issajenko. Gareau also won relay medals at the 1990 Commonwealth Games, 1993 Summer Universiade and at two Francophone Games.