He was the son of Alphonse E. Gagnon, a milling manager, and a cultured English mother, who was interested in literature.

Early in life, his mother had encouraged him to learn drawing and painting, but his father wanted him to become a businessman.

While there, he painted the French countryside and the beaches at Saint-Malo, Dinan, and Dinard in small oil-on-boards and canvases, lightening his colour palette and recording the effects of light.

[2] Gagnon showed his early promise by winning a bronze medal at the Canadian exhibition at the Louisiana Purchase Exposition in 1904.

[3] Before returning to Canada in the autumn of 1907, he travelled to Spain, Italy, England, and Norway making sketches for paintings and prints.

In 1925 and 1926, he provided original designs for the Christmas cards of the Canadian Artists Series published by Rous and Mann Limited of Toronto.

At the Cowley Abbott Auction of An Important Private Collection of Canadian Art – Part III, December 6, 2023, Lot #117, Gagnon's Ice Harvest, Quebec, 1935, oil on canvas, 25 x 36 ins ( 63.5 x 91.4 cms ), Auction Estimate: $400,000.00 - $600,000.00, realized a price of $984,000.00.