The loss is the Lancers' fifth in their past six games as they fall to 16-8 overall, 10-7 PacWest.
The 'Lopes, meanwhile, win their eight straight and improve to 18-7, 12-5.
Luke Evans led all players with 22 points, moving within 25 points of 1,000 for his career. He was the only Lancer in double figures and also grabbed eight rebounds.
The Lancers never trailed as GCU, which climbed into the regional rankings this week at No. 10 and are on the bubble for the national tournament, took the lead for good on a Nick Witherill 3-pointer with 11:45 left in the first half that gave the 'Lopes a 14-11 lead. The Lancers got within 22-21 on a Kevin Deeb 3-pointer, but the 'Lopes scored seven straight, five from Witherill to open a 29-21 lead with 3:45 to play.
Ivan Patterson got the Lancers within 55-52 with a three-point play with 7:32 to play, but Kyle Speed answered right back with a 3-pointer on the ensuing possession and started 14-5 run that put the game away and gave GCU a 69-57 lead with two minutes left.
Five 'Lopes scored in double figures. Witherill came off the bench and scored 15 points. Jerome Garrison and Braylon Pickrel each had 13 points. Pickrel grabbed a game-high 10 rebounds.
Grand Canyon shot 52.6 percent in the second half and finished the game 23-for-46 (50 percent) while holding CBU to 39.3 percent. The Lancers were 7-for-14 from beyond the arc.