Elias Harris had 19 points for the top-seeded Bulldogs (31-2), who defeated Saint Mary's for a third time this season and completed an undefeated schedule in conference play while racking up their fifth consecutive WCC crown.
The victory was Gonzaga's 14th straight as it heads into the NCAA Tournament with a potential No. 1 overall seed.
Stephen Holt provided 19 points for the second-seeded Gaels (27-6), who managed just 36 percent shooting from the field.
Saint Mary's leading scorer, Matthew Dellavedova, was held to two points on 1-of-8 shooting.