He played in Major League Baseball (MLB) for the New York Mets, Baltimore Orioles and Tampa Bay Rays.

Prior to beginning his professional career, Gilmartin attended Florida State University, and was an All-American pitcher for the Seminoles baseball team.

After graduating from high school, the San Diego Padres selected Gilmartin in the 31st round of the 2008 Major League Baseball Draft.

[7] After the 2013 season, the Braves traded Gilmartin to the Minnesota Twins for Ryan Doumit on December 18, 2013.

During the 2014 Winter Meetings, the New York Mets selected Gilmartin from the Twins in the Rule 5 draft on December 11.

[10] Gilmartin competed to make the Mets' Opening Day 25-man roster as a relief pitcher and earned a spot in the bullpen.

[11] Gilmartin made his major league debut on April 10 against the Braves in a 5–3 loss, getting Nick Markakis to ground out and striking out Freddie Freeman to end the seventh inning.

[12] On June 14, Gilmartin got his first win, against the Braves, holding Atlanta scoreless through both the fifth and the sixth innings.

Gilmartin appeared in game two of the World Series in the top of the eighth inning, retiring the two batters he faced.

[22] Gilmartin had pitched five scoreless innings in two games during his brief call-up, giving up two hits and striking out five batters.

[26] After starting the 2019 season in the minor leagues for the Orioles, Gilmartin had his contract selected to the majors on June 17, 2019.

On February 8, 2020, Gilmartin signed a minor league deal with the Tampa Bay Rays.

[32] With the 2020 Tampa Bay Rays, Gilmartin appeared in two games, compiling a 0–0 record with 8.31 ERA and five strikeouts in 4.1 innings pitched.

[34] On April 27, 2021, Gilmartin signed with the Long Island Ducks of the Atlantic League of Professional Baseball.

[36][37] In 6 games split between Wichita and the rookie-level Florida Complex League Twins, he struggled to a 14.63 ERA with 13 strikeouts in 8 innings pitched.