Gregory made three 3's in a row for the Grizzlies to close out the first half, but Montana still trailed by seven points at intermission. The Grizzlies opened the second with a 13-2 run - Gregory accounting for eight points with two 3's and a layup - to take the lead with 13:51 remaining.
The Lumberjacks threatened with 3:22 left after Gaellan Bewernick scored to make it a five-point game. The Grizzlies were 7 of 8 from the line - Gregory 5 of 6 - to end it.
Gregory finished with 31 points and shot 70 percent from 3-point range. Martin Breunig added 16 points for Montana (9-8, 5-1 Big Sky).
Quinton Upshur scored 20 points to lead Northern Arizona (8-11, 3-3).