On a grant from the Québec government she went to the Juilliard School, New York, to study with soprano Muriel Tannahill (the wife of Jean Dansereau). On 31 May 1945 she made her stage debut at His Majesty's Theatre, Montreal, as Susanna in The Marriage of Figaro with artists from the Metropolitan Opera. On 2 Jun 1945 she sang Micaëla in Carmen at the Montreal Festivals. In Canada and the US she took part in many concerts with such artists as Grace Moore, Witold Malcuzynski, André Kostelanetz, and Percy Faith. She appeared on numerous CBC radio and TV opera presentations, notably La Bohème (1955, 1961), Così fan tutte and L'Enfant et les sortilèges (1956), Benjamin's Prima Donna (1957), and Madama Butterfly (1958). She repeated Susanna (1956) for the Montreal Festivals and sang Zerlina in Don Giovanni (1957) at the St-Denis Theatre.
Gagnier was a regular guest on the CBC radio program Serenade For Strings with conductor Jean Deslauriers, and on the CBC television programs À la claire fontaine and The Jackie Rae Show. She performed operetta, musical comedy, and song which perfectly suited her voice and personality. In 1967 she took the role of Rési in a series of performances of Valses de Vienne at the Salle Wilfrid-Pelletier of the PDA. The St-Jean-Baptiste Society of Montreal awarded her its 1972 Prix de musique Calixa-Lavallée. In 1974 she again sang the role of Zerlina in Montréal and Québec City with the Opéra du Québec. Gagnier's voice was distinguished by a clear and luminous timbre, effortless high notes, and a range that allowed her to take on coloratura as well as lyric roles. These qualities and her personal charm made her one of the best loved singers in Québec.
In 1990 Gagnier was invested as a Member of the Order of Canada. She was inducted into the Canadian Opera Hall of Fame in 1996.
See also J.-J. Gagnier, Guillaume Gagnier, René Gagnier, Armand Gagnier, Ernest Gagnier, Lucien Gagnier, Réal Gagnier, Roland Gagnier, Gérald Gagnier, and Ève Gagnier (all members of the same musical family).
- 12 Meilleures Chansons canadiennes du concours "Chansons sur mesure." 1962. Variétés V-9000
- Puccini "Musetta's Waltz" La Bohème - Chopin Tristesse éternelle. J.-P. Morel O. Ca 1948. RCA 10-1304 (78)
- O. Straus "Mon Héros" The Chocolate Soldier; Rêve de valse. J.-P. Morel O. Ca 1948. RCA 10-1305 (78)