[3] William Nicoll Cresswell, an artist who lived near Seaforth, began to teach him drawing and painting in 1863.

[3][5][6] Gagen painted sea and river studies and travelled widely to do so, favoring the Maritime Provinces and the shore of St. Lawrence in Quebec as well as Maine and Massachusetts in the United States.

[3] In 1918, the Canadian War Memorials Fund asked him to record the shipbuilding underway in Toronto, Ontario.

[12] In 1893, he was appointed one of the Canadian Fine Arts Commissioners to the World’s Columbian Exposition, Chicago.

1919, a typed manuscript in the collection of The Edward P. Taylor Library & Archives of the Art Gallery of Ontario in Toronto.

The Toronto Daily Star report of the funeral said, "The assemblage of so many painters, architects, writers, musicians, sculptors and university professors as well as associate members demonstrated how widespread was the influence of this remarkable old artist whose art and mentality had always kept the vigor of eternal youth.