There was some discussion on this game in the catch-all weekly games thread:
Games Feb. 3-9
St. Mary's shot the game of its life, 14 of 21 from 3, and as you say Gonzaga came out ice cold.
More important than how it affects Gozaga's ranking is the question of whether this drops Gonzaga from hosting status. They were #12 in last Monday's reveal, but their actual resume is awfully thin -- only top 50 wins are over Missouri State and Purdue -- and I don't know that they could afford a really bad loss.