Lamar (4-3) picked up back-to-back victories for the first time this season.
Idaho State (2-4), which now has lost four straight, led most of the first half but Gregory scored five points late and Lincoln Davis closed with a dunk to give Lamar a 39-36 halftime advantage.
Idaho State's Kyle Ingram opened the second half with a 3-point play to take a 43-42 lead. There were ten lead changes in the first 10 minutes of the second period until Davis, who finished with 16 points, hit two straight jump shots and Gregory had a layup for a 63-59 lead with 9:24 left. The Cardinals never trailed again.
Ali Faruq-Bey and Ingram led Idaho State with 16 and 15 points respectively.