BYU (17-10, 9-5 West Coast Conference) jumped out to 9-0 lead but Pacific responded by scoring 17 of the next 21 points to take a four-point lead with just fewer than 11 minutes to play in the first half.
The Cougars answered by scoring seven straight before taking a 38-35 lead into the break as the Tigers (14-10, 5-8) shot just 41 percent in the first 20 minutes.
Gill scored 12 points in the second half as Pacific made 15 of 24 field goals, outscoring BYU 54-44 after halftime.
Matt Carlino paced BYU with 28 points and Tyler Haws, the nation's third-leading scorer at 24.5 points per game, finished with 20.
Trevin Harris scored 13 for Pacific.