When the Mets made the playoffs, Gilmartin was not on the roster for the Division Series, but was added to the roster for the Championship Series, replacing Erik Goeddel. Gilmartin did not appear in the championship series. Gilmartin appeared in game two of the World Series in the top of the eighth inning, retiring the two batters he faced. Gilmartin finished the 2015 regular season with a record of 3-2, and a 2.67 ERA in 50 games (one start) in innings pitched, with 54 strikeouts and a WHIP of 1.186, while giving up 50 hits, 17 runs, two home runs, and 19 walks.