Gallo was brought up from Double-A Frisco before Tuesday's game against the Chicago White Sox. He was batting sixth and playing third base in place of injured Adrian Beltre, who is out with a sprained left thumb.
Smolinski, who was sent down to Triple-A Round Rock last Wednesday, was the move for Beltre's addition to the DL, effective Monday.
Left-handed reliever Alex Claudio was optioned to Round Rock to make room on the roster for Gallo.
The Rangers also said left-hander Matt Harrison (spinal fusion surgery) and outfielder/first baseman Ryan Rua (right heel) would start rehab assignments Wednesday with Round Rock.