Gearrin was traded from the Rangers to the Athletics on Friday in exchange for minor-league pitchers Abdiel Mendoza and Teodoro Ortega.
Gearrin was acquired by the Rangers from the Giants in early July and has since posted a 2.53 ERA and 0.89 WHIP over 21.1 innings. The Athletics' stacked bullpen gains another quality arm, as Gearrin is likely to slide into a middle-relief role.