Gagnon was chosen by the Milwaukee Brewers in the third round of the 2011 MLB draft. He spent six seasons in the Brewers organization before being traded to the Angels in December 2016. He became a free agent after the 2017 season and the Mets signed him to a minor league contract in the offseason. He got his first shot at the big leagues in 2018, posting a 5.25 ERA in 12 innings for the Mets.
He logged nearly twice as many big league innings in 2019 for the beleaguered Mets bullpen, but things did not go well. He put up an unsightly 8.37 ERA and 8.96 FIP in 18 games. His chief problem was the home run ball; he gave up an alarming 22 home runs in 23 ⅔ innings, something he will need to control if he can hope to have success in the small ballparks of the KBO League. He managed to post -1.0 fWAR during his up-and-down 2019—an impressive feat in so few innings.
Gagnon was out of options for the Mets, which means his opportunities in Queens were likely limited. The Mets designated Gagnon for assignment last month in order to free up space on the 40-man roster to protect other players from the Rule 5 draft.