Top-seeded St. Mary's earned the conference's automatic berth in the NCAA Tournament in a dramatic conclusion to the first league championship game this year involving two ranked teams.
The Gaels and Bulldogs split their two regular-season meetings. Gonzaga came into this contest ranked 25th and St. Mary's had a ranking of 18th.
St. Mary's had a five-point lead late in regulation, but Elias Harris made a layup for Gonzaga with 17 seconds to go and then forced overtime by hitting a 3-pointer with 4 seconds left.
Dellavedova broke a tie by making two free throws for the Gaels with 39 seconds left in the extra period and Page boosted the advantage to four points by making two foul shots with 24 seconds remaining.
After Sam Dower scored on a layup for Gonzaga, Dellavedova put the game away with two more from the line.
Dellavedova scored 22 points and Rob Jones added 18 for the Gaels (27-5). Harris went 10-of-14 from the field and scored 22 points for Gonzaga (25-6).