The Braves also agreed to terms on a minor league deal with utility player Emilio Bonifacio, who was assigned to Gwinnett. Bonifacio was designated for assignment by the Braves on April 2.
Withrow, 27, missed all of 2015 while recovering from elbow and back surgeries. He was 3-0 with one save and a 2.73 ERA in 46 relief appearances for the Los Angeles Dodgers in 2013-14.
Withrow retired the only batter he faced at Gwinnett. He said Sunday it was good "to get the blood flowing" in a game. He called it a "relief" to be back in the majors.
Gant, 23, gave up three hits and two runs in one inning in the Braves' 12-2 loss to St. Louis on Saturday night. He had a 12.00 ERA in three games.
Gant will work as a starter at Gwinnett.