Robert John Gsellman (/ɡəˈzɛlmən/ gə-ZEL-mən;[1] born July 18, 1993) is an American professional baseball pitcher who is a free agent.

He was a California Interscholastic Federation state champion in basketball as a junior and voted team captain as a senior.

[10] During 2015 with St. Lucie, he was named to the FSL mid-season All Star team, however, did not participate due to his promotion to Binghamton.

Taking over for Jonathan Niese, who had left the game with left knee pain after pitching 1⁄3 of an inning, Gsellman pitched 3 and 1⁄3 innings, racking up 2 strikeouts, giving up no runs, and earning his first Major League win, the second Met rookie to do so in his debut in 2016, after Gabriel Ynoa.

[15] Nevertheless, on September 25 at Citi Field, Gsellman picked up his first Major League hit on a bunt off of Phillies pitcher Jake Thompson.

[20] In 2019 for the Mets, Gsellman pitched to a 4.66 ERA in 63.2 innings across 52 appearances before suffering a partially torn right lat in August that would cause him to miss the remainder of the season.

[21] Gsellman opened the pandemic shortened 2020 season on the injured list due to right triceps soreness.

On September 8, 2020, Gsellman suffered a fractured rib in an appearance against the Baltimore Orioles, and missed the remainder of the 2020 season.

[26] He was assigned to the Triple-A Iowa Cubs to begin the season, pitching to a 1.17 ERA with 12 strikeouts in 5 appearances.