Montana (14-10, 10-3 Big Sky) trailed by three points in the final seconds of the second overtime when Gregory rebounded his own missed 3-point attempt and air balled another 3. But Martin Breunig tapped the missed shot back out to Brandon Gfeller, who nailed the game-tying 3-point attempt at the buzzer.
Gfeller had 21 points and Gregory had 20 to lead the Grizzlies, who moved into a second-place tie with Sacramento State.
Northern Colorado (12-12, 7-6) had its largest lead at 64-59 with 1:40 left in the first overtime.
The Bears trailed 33-18 late in the first half but had a 12-0 run midway through the second to tie it at 48. The Grizzlies pushed back in front at 57-52, but went scoreless for the final 4:26 as Northern Colorado scored the last five points in regulation to force the first overtime.
Tevin Svihovec led the Bears with 23 points.