Gallo, the Rangers' top prospect according to MLBPipeline.com, was hitting .265 with eight home runs, 20 RBIs and a .639 slugging percentage in 24 games at Round Rock, despite missing 18 days because of a groin strain. He can be used defensively at first, third and the corner-outfield spots.
Choo is expected to miss 3-4 weeks. He was sidelined for 40 days with a right calf strain before being activated on Friday against the Astros. He had two plate appearances, walking both times, before injuring the hamstring muscle running the bases.
Stubbs was injured running the bases in the ninth inning of Sunday's game against the Astros. He was signed by the Rangers as a free agent on May 7 after being released by the Braves.
Choo was hoping to be back in the lineup but an MRI showed the strain to be worse than expected. Choo was told there was a chance he could injure the hamstring even more playing on it and be out 6-8 weeks.
"I'm mad at myself," Choo said. "Come back from the disabled list and this happens. Nobody wants to get hurt. It's frustrating."
Hoying, a left-handed hitter, was a 10th-round pick out of the University of Toledo who has been in the Rangers' farm system for seven seasons. In his fourth season at Round Rock, he was hitting .319 with seven home runs, 32 RBIs, a .409 on-base percentage and .550 slugging.
"He has had a tremendous first month-and-a-half of the season," general manager Jon Daniels said. "Not only the power and the plate discipline, he also runs the bases well, plays great defense. It was an easy recommendation for our guys."
The Rangers elected to call up Hoying rather than Delino DeShields, who was optioned to Round Rock on May 13.
"No. 1, the way Jared has played, it's an easy call," Daniels said. "No. 2, replacing Choo, a little left-handed power with Jared made sense. Third, we sent Delino down to work on some things and we want to let him continue to work on things."
Gallo, 22, was with the Rangers for 36 games last season, hitting .204 with six home runs, 14 RBIs and a .417 slugging percentage.
• Second baseman Rougned Odor is scheduled to have his appeal heard on Tuesday. Odor was suspended for eight games for punching Blue Jays outfielder Jose Bautista, and Daniels said, "We hope it gets reduced a little bit, but needless to say, he'll serve a suspension."
• Daniels said if the Rangers do make a move to add an extra infielder while Odor is suspended, it will be Jurickson Profar. The Rangers can't replace Odor on the 25-man roster while he serves his suspension but they can call up Profar and send down a reliever.
• Catcher Ivan Rodriguez will represent the Rangers at the Major League Draft on June 9 in Seacaucus, N.J.