Gantz missed a 44-yard attempt on the Bearcats' previous possession, but Cincinnati's defense forced a three-and-out.
Gunner Kiel drove Cincinnati (7-5, 4-4 American) to the Pirates' 25-yard line. Gantz split the uprights for his second game-winning field goal against East Carolina in as many years. Kiel finished 30 of 47 for 270 yards with a touchdown and interception.
Cincinnati took the lead on its opening drive of the second half when Mike Boone capped off a 16-play drive with a 1-yard plunge, giving the Bearcats a 16-10 lead.
After Gantz hit a 50-yarder, East Carolina (5-7, 3-5) tied it at 16-all when James Summers found Bryce Williams for a 2-yard touchdown. But Summers was called for unsportsmanlike conduct after the play - a 15-yard penalty - and Davis Plowman missed the longer extra point try.