Gonzaga owned only a two-point lead with 9:50 to go, but then left the Gaels behind and ended St. Mary's 13-game winning streak. It was the first loss in West Coast Conference action for the Gaels, who had their league lead over Gonzaga trimmed to 1 1/2 games.
The Bulldogs won for the 14th time in 16 games, hitting 53 percent from the field. Gonzaga lost its previous meeting with St. Mary's this season 83-62.
Robert Sacre and Gary Bell both scored 12 points for Gonzaga (19-4, 9-2 WCC).
St. Mary's got 20 points from Matthew Dellavedova, who made only 5-of-12 shots from the field. The Gales (22-3, 11-1) shot a mere 38 percent. It was the first loss for St. Mary's since a Dec. 22 setback against Baylor.