Jordan Gregory scored 20, Martin Breuning added 13 and Mario Dunn had 11 points for Montana (6-8, 3-1 Big Sky), which made 29 of 52 (56 percent) field goals, including a season-high 15 3-pointers on 27 attempts.
Gfeller made three-straight 3-pointers to make it 9-0 fewer than 2 minutes in, and his fourth 3 gave the Grizzlies a 10-point cushion with 15:07 left in the half.
The sophomore guard hit another trey just over a minute later, sparking a 14-3 run which blew the game open.
Montana led by as many as 22 in the first half and 30 in the second.
Marcus Bradley led Idaho State (4-12, 1-3) with 19 points.
The Bengals shot 47 percent, but made just 3 of 14 from deep and 7 of 17 free throws.