A businessman, he served as a legislator for both the National Assembly of Quebec and the House of Commons.
Gagnon ran as a Parti Québécois candidate in 1976 in the provincial riding of Champlain.
He entered federal politics when he was elected as a Member of the Bloc Québécois in 2000 in the riding of Champlain, with 45.3% of the vote against Liberal Julie Boulet (45.2%).
During his tenure, he served as the Bloc critic to Seniors and proposed that eligible recipients be signed up to the Guaranteed Income Supplement Program (GIS) without having to make a request.
At one point, Gagnon was critic to the Library of Parliament, the Minister of Agriculture and Agri-Food.