After Gardner's touchdown, Michigan linebacker Kenny Demens tackled Northwestern tailback Tony Jones to end the Wildcats' only overtime drive.
Brendan Gibbons' 26-yard field goal with eight seconds left in the fourth quarter forced overtime, and capped off a second-half rally for the Wolverines, who trailed 24-14.
Michigan (7-3, 5-1 Big Ten) hosts Iowa at noon Nov. 17 at Michigan Stadium in its final home game.