Kevin Pangos chipped in 14 points for the Bulldogs (30-2), who notched their 13th straight win and first-ever victory as the nation's top-ranked team.
Kelly Olynyk and Gary Bell Jr. provided eight points apiece in the WCC tournament semifinal triumph.
Gonzaga will face Saint Mary's in Monday's title game, in which the Bulldogs will be seeking their 12th conference championship since 1995.
Saint Mary's beat San Diego in overtime in Saturday's other WCC semifinal.
The Lions (11-23) were led by Anthony Ireland with 19 points and six rebounds.