Toronto has won back-to-back games for the first time this season and has the lead in the Atlantic Division despite owning a 6-7 record.
Gay scored 17 points overall and dominated down the stretch after Washington had closed within three points with less than three minutes to play.
DeMar DeRozan also had 17 points for the Raptors and Amir Johnson had 14.
Washington, which had won its previous two games, wasted a tremendous showing by John Wall -- who scored 37 points while hitting 15-of-21 from the field.
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