After playing three games in the minor leagues to correct his swing, Gattis returned to the majors and made an immediate impact.
"I felt fixed before leaving," Gattis said. "It's good to go there and really kind of put it into action. It was good to have a good night."
The Braves have won 20 of 24 at Turner Field to improve the major leagues' best home record to 51-19.
Gattis, who was recalled earlier in the day after a three-game stint at Triple-A Gwinnett, tied it at 1-all with his 16th homer in the seventh.
Medlen (12-12) allowed seven hits and one run while striking out nine.
Closer Craig Kimbrel converted his 34th straight save opportunity and improved to 44 for 47 this season. He struck out Juan Lagares and Matt den Dekker, allowed a single to Omar Quintanilla and retired pinch-hitter Justin Turner on a groundout.
New York has dropped three straight and nine of 13.
Torres gave up seven hits, three runs and one walk. He tied career highs with six strikeouts and seven innings pitched.
In the seventh, New York manager Terry Collins let Torres bat with runners at first and third, but the pitcher struck out.
Torres lost the lead in the bottom half of the inning, unable to finish off Gattis and Uggla when both hitters fell behind with 0-2 counts.
"Look where he was in that point in the game, and he was cruising," Collins said. "They're dangerous so you can't make any mistakes. He gave us a chance. We just couldn't put up any runs."
Medlen erased the first two runners in scoring position -- Eric Young Jr. in the first and Duda in the fourth.
After Young tripled over Gattis' head in left field in the sixth and scored on Murphy's double, Medlen escaped further damage when Satin grounded out and Duda took a called third strike.
"Whatever jam you get in, you always feel like you can get out of it," Medlen said. "I'm always thinking positive. Whether other people do, I don't know, but I'm always figuring out a way to get out of stuff. It's something you learn as you mature in this game. I think I've figured how to battle, how to go at guys, and today I did a decent job of that."
Duda doubled with two out in the fourth, getting stranded on d'Arnaud's strikeout, but his average dropped to .169 in 59 at-bats with runners in scoring position.
Gattis, who went 3 for 3, spent the previous three days at Triple-A Gwinnett to get some at-bats. The NL's rookie of the month winner for April and May had slumped in August, but he seemed to correct his issues by going 6 for 13 in three games at Gwinnett.
Gattis took four pitches from Torres before crushing a cutter into the seats in left-center field. It was his first homer since the Braves' 8-2 victory at New York on July 24 -- a span of 70 at-bats.
"It's good to square the ball up, first," Gattis said. "It just ended up being a home run, you know."
Atlanta has won 29 of its last 38 games and has rallied for 41 victories, best in the NL.
Braves manager Fredi Gonzalez said RF Jason Heyward, on the 15-day disabled list with a broken jaw, met with a doctor Tuesday and has been cleared to play catch and hit balls off a tee. ... Medlen is 2-1 with a 3.20 ERA and has 29 strikeouts in 25 1/3 innings in four starts against the Mets this year. ... Collins said before the game that he expects RHP Daisuke Matsuzaka will make his next scheduled start Sunday at Cleveland. Matsuzaka is 0-3 with a 10.95 ERA in three starts with New York. ... Collins added 1B Ike Davis, sidelined for the fourth straight with a strained right oblique, was in New York to see a doctor. ... Braves OF Jordan Schafer grounded out as a pinch-hitter for Medlen in the seventh. Schafer went 4 for 5 on Monday before leaving the game early with a sore back.