The team announced the roster move and optioning left-handed pitcher Kyle Bird to Triple-A Nashville.
Gallo was out with a left oblique strain he suffered June 1 against the Kansas City Royals. He was hitting .276 with 17 homers and 41 runs batted in when he was injured.
Gallo was tied for the American League lead in home runs when he suffered the injury. He also is one of the finalists in the 2019 MLB All-Star Starters Election.
The center fielder hit 40 homers with 92 runs batted in during 148 games in 2018.