Gay, who was part of a Jan. 30 trade that sent him to the Raptors after spending his first 6-plus years in the league with Memphis, was 8 of 18 from the field.
DeMar DeRozan scored 13 of his 18 points in the first quarter.
Mike Conley led Memphis with 29 points. Marc Gasol had 18 points while Zach Randolph finished with 10 points. Gasol and Randolph grabbed 10 rebounds each.
Memphis outrebounded the Raptors 40-37 and led the points in the paint 36-24. But the 8 of 15 from outside the arc for Toronto, plus hitting 33 of 39 free throws compared to 11 of 17 for Memphis gave the Raptors the advantage.
Gay, who missed his first four shots in the game, connected on his next four in the second quarter, including three outside the arc. That helped the Raptors lead by as many as 15 in the first half before carrying a 55-44 halftime advantage.
DeRozan had 14 to lead Toronto at that point, while Lowry already had 11. The Raptors hit 7 of 9 shots outside the arc before the break.
Conley had 11 points and Allen scored 10 for Memphis, which had its turnover problems return with nine miscues in the half. Memphis ended with 19 in the game.
Conley got Memphis untracked early in the third scoring 10 early points to key a Memphis run. But the Grizzlies couldn't initially overtake the Raptors as Toronto weathered the initial defensive intensity to start the second half.
Memphis did pull even at 70 on a steal and layup by Conley, but the Raptors closed the stanza on an 8-1 rally for a 78-71 lead after three.
The Raptors extended that lead to 88-74 with a 10-3 run opening the fourth quarter. Memphis' 16th turnover led to a dunk by Terrence Ross, and Grizzlies coach Dave Joerger called timeout. It didn't do much good because the Raptors would hit two more baskets before Mike Miller's 3-pointer stopped the run.
When Gay's 19-footer ran the score to 96-77 Raptors, the Memphis crowd began to boo.
NOTES: Gay is still the career leader in games played and ranks second in points and field goals made for the Grizzlies. .A lot was made of Gay's return, but Austin Daye, who played with Memphis last season after the Gay trade and Lowry, who started his career in Memphis, are part of Grizzlies history. ... Memphis G Jerryd Bayless returned to action after missing two games with a right knee sprain. Memphis F/G Quincy Pondexter missed the game recovering from a broken nose. ... The loss was the second consecutive for the Grizzlies. Memphis had alternated wins and losses through the first seven games of the season.