[2] In 2011, as a junior at Long Beach State, he was 4–10 with a 2.81 ERA in 15 starts, earning him a spot on the All-Big West Second Team.

[7] He spent the 2017 season with the Salt Lake Bees where he compiled a 1–1 record and 6.25 ERA in 31 games (ten starts).

[12] Gagnon made his Major League debut in a July 10 start against the Philadelphia Phillies at Citi Field.

[13] Coincidentally, the starting pitcher for the Phillies in that game was Enyel De Los Santos, who was also making his Major League debut and earned the win for New York.

On December 26, 2020, Gagnon signed with the Wei Chuan Dragons of the Chinese Professional Baseball League for the 2021 season.

In 2023, Gagnon made 30 starts for Wei Chuan, posting a 13–7 record and 3.00 ERA with 155 strikeouts across 183 innings of work.