[1] Born in L'Ancienne-Lorette, Quebec, Gagnier moved to Montreal in 1874 after deciding to pursue a music career.
In that city he studied the clarinet with Oscar Arnold and Jacques Vanpoucke.
[1] Gagnier began his career performing in the orchestras of a variety of Montreal theatres during the late 1870s and 1880s.
Goulet's Montreal Symphony Orchestra, following the ensemble's unsuccessful attempt to acquire two bassoonists among the city's other musicians.
He taught music and a variety of instruments to most of them, "thus forming a veritable Gagnier orchestra".