Gagner played midget hockey with the toronto marlboros and scored 173 points. Gagner decided to play major junior hockey in canada. He was considered a 1st round talent and was drafted 4th in the ohl entry draft by the London knights he then began he ohl career playing 53 games, and getting 118 points. Gagner was on a line with patrick kane who plays for the black-hawks now and sergi kostiston who plays on the nashville predators.  

Gagner was one of the best in the shootout, his move was almost unstopable. Gagner used this in the ohl and in the nhl when he was on the oilers.