Gagnon was born in Cap-Sainte-Anne (now Chicoutimi North). He was a self-taught artist, who first exhibited in Arvida. He then lived in New York City until he moved to Montreal in 1967 and founded the Galerie des peintres canadiens at the Place des Arts. In 1974, he exhibited at the Bernheim-Jeune gallery in Paris thanks to music producer Gilles Talbot and journalist Jacques Ourévitch. He had two other solo exhibitions in Paris in 1977 and 1984. played piano and clarinet, and developed an enduring love of jazz, frequenting the nightspots in Midtown to hear it.