He played in Major League Baseball (MLB) for the Atlanta Braves, San Francisco Giants, Texas Rangers, Oakland Athletics, Seattle Mariners, New York Yankees, and Minnesota Twins.

Prior to playing professionally, Gearrin attended Rhea County High School, Young Harris College, and Mercer University.

He began his professional career in 2007, going 1–1 with a 4.44 ERA in 18 relief appearances, striking out 37 batters in 261⁄3 innings of work for the Danville Braves.

The following season, he went 6–3 with a 4.11 ERA in 36 relief appearances for the Rome Braves and Myrtle Beach Pelicans, striking out 72 batters in 46 innings pitched.

[6] On April 22, 2011, Gearrin was called up to Atlanta's major league team from AAA Gwinnett, a move that General Manager, Frank Wren, described as a "necessity."

In the bottom of the ninth of a tied game against San Diego on April 25, 2011, Gearrin made his major league debut by retiring the side in order and getting a strikeout.

On May 1, 2011, Gearrin recorded his first blown save after giving up the tying run in the 7th inning to the St. Louis Cardinals, a game the Braves later came back to win.

On July 8, 2018, Gearrin was traded to the Texas Rangers along with Austin Jackson and minor league pitcher Jason Bahr in exchange for a player to be named later or cash considerations.

[13] On August 31, 2018, Gearrin was traded to the Oakland Athletics in exchange for minor league pitchers Abdiel Mendoza and Teodoro Ortega.

On February 17, 2020, Gearrin signed a minor league deal with the Minnesota Twins that included an invite to Spring Training.