He was born in La Malbaie, Canada East, the son of Jean Gagné and Christine Blackburn[1] and was educated at the Seminaire de Québec. Gagné was called to the Lower Canada bar in 1864 and set up practice in Chicoutimi.[1] In 1864, he married Marie Emilie Louise Guay, the sister of Joseph-Dominique Guay. Gagné was an unsuccessful candidate in the 1872 and 1878 federal elections. He was defeated when he ran for reelection in 1887. In 1889, he was named a judge in the Quebec Superior Court for Chicoutimi district. He died in Quebec City at the age of 68.[1]
