Guyn's bucket sent the game to overtime and his jumper with 3:18 left broke a tie and started an 11-1 game-ending run.
Banks added 11 rebounds. T.J. Cromer and Deuce Bello both scored 14 points and Peter Jurkin added 11 for ETSU (13-7, 6-1 Southern), which shot 54 percent. The Bucs moved into a tie for first with Chattanooga.
Gyun tied the game at 86-all with 14 seconds left in regulation. Sledge was fouled but missed the front end of a 1-one-1 and the game went to overtime.
The Bulldogs and Bucs, the conference's top two scoring teams, were a combined 74-of-156 shooting.