Gonzaga used 30 points from freshman Kevin Pangos to cruise past Brigham-Young 77-58 in one of the semis.
Pangos drained five 3-pointers and went 10-of-17 from the field overall, while Elias Harris contributed 10 points and 15 rebounds for the Bulldogs (25-5).
Gonzaga led 38-26 at the half and went on to secure its 15th straight appearance in the WCC title game.
Brandon Davies led BYU (25-8) with 17 points.
Saint Mary's, meanwhile, used six treys from Clint Steindl to help down San Francisco 83-78 in Saturday's first semifinal.
Steindl finished with 19 points and all five starters for the Gaels (26-5) reached double figures.
Cody Doolin paced the Dons (20-13) with 26 points.