Tretter injured his left knee Aug. 22 in an exhibition game against Oakland. He will be eligible to return for the Packers' Nov. 9 game against Chicago.
Gerhart, a 6-foot-1, 310-pound former Arizona State player, was on Green Bay's practice squad at the end of the 2012 season and the final 12 weeks and playoff game in 2013. He also has spent time on practice squads with Cleveland and Carolina.
The Packers also signed guard Josh Allen to the practice squad. Allen, a 6-foot-3, 312-pound rookie from Louisiana-Monroe, was cut by Tampa Bay on Sunday.