Although they have not thrown an MLB pitch in 2023, Robert Gsellman threw 6,171 pitches that were tracked by the PITCHf/x system between 2016 and 2022, including pitches thrown in the MLB Regular Season and Spring Training. In 2022, they relied primarily on their Sinker (93mph) and Slider (90mph), also mixing in a Change (85mph). He also rarely threw a Curve (81mph) and Fourseam Fastball (93mph).
In 2022, compared to other RHP:
His sinker has heavy sinking action, has an obvious tail, generates more whiffs/swing compared to other pitchers' sinkers and results in somewhat more flyballs compared to other pitchers' sinkers. His slider is thrown extremely hard, is a real worm killer that generates an extreme number of groundballs compared to other pitchers' sliders, has primarily 12-6 movement and has less than expected depth. His change results in more flyballs compared to other pitchers' changeups, is slightly firmer than usual, has slight armside fade and has some natural sink to it. His curve (take this with a grain of salt because he's only thrown 7 of them in 2022) is basically never swung at and missed compared to other pitchers' curves, results in more flyballs compared to other pitchers' curves, has a sharp downward bite, is slightly harder than usual and has slight glove-side movement. His fourseam fastball (take this with a grain of salt because he's only thrown 3 of them in 2022) generates an extremely high number of swings & misses compared to other pitchers' fourseamers, has heavy sinking action, is an extreme flyball pitch compared to other pitchers' fourseamers, has essentially average velo and has slight armside run.