Gattis delivered his second game-winning hit of the series, this time producing a two-run pinch double with two outs in the eighth inning to give the Braves a 3-1 victory at Turner Field.
"It's a luxury to give him a day off and have him lurking there the whole game," manager Fredi Gonzalez said of his catcher, who is 7-for-13 as a pinch hitter.
Gattis won the series opener on Monday with a walk-off home run in the 10th inning and has a .377 average with seven doubles, four homers and 17 RBIs in 46 career at-bats against Miami.
"I know I got off to a good start against them early, the first series last year, and hit a couple of home runs," Gattis said of the start to his success against the Marlins.
Gattis' decisive hit this time came off right-hander A.J. Ramos after lefty Mike Dunn had given up a walk to third baseman Chris Johnson and infield single to second baseman Dan Uggla.
"Sometimes when you come off the bench you get anxious," Gattis said. "You get only one crack at it and fortunately I was able to come through."
The right-handed hitter turned around a 1-1 fastball and lined a shot into the left-field corner.
"I was trying to be loose against a guy with a lot of velocity," Gattis said.
"I have faith in my fastball," Ramos said. "It was just a bad pitch over the middle."
Braves closer Craig Kimbrel blew his first save of the season in the series opener, but retired the Marlins in order in the ninth inning this time, striking out two.
Both starting pitchers were dominant through six innings, with the run off the Marlins' Nathan Eovaldi unearned.
Braves starter Aaron Harang, who came in leading the National League with a 0.70 ERA, struck out 11. The veteran right-hander's only walk came to the leadoff batter in the seventh and he gave up six hits.
"I'm trying to stay aggressive and pound the strike zone," said Harang, who was picked up at the end of spring training. "I couldn't have told you how many strikeouts I had."
Eovaldi allowed five hits, walked one and struck out seven while lowering his ERA to 2.87 and continuing his stellar work against Atlanta. He had a 0.62 ERA against the Braves over his previous four starts.
"I feel that I know these guys pretty well and I was able to locate my pitches," Eovaldi said.
Harang took a no-hitter into the seventh at Milwaukee on April 2 and had a no-hitter through seven innings against the Mets in New York last Friday before being pulled after 121 pitches and six walks.
Harang retired the first six Marlins to stretch his no-hit streak to nine innings before shortstop Adeiny Hechavarria led off the third with a ground single up the middle.
Hechavarria was caught attempting to steal second and Miami didn't have a second base runner until second baseman Derek Dietrich bounced a single into right field with two outs in the fifth.
The Marlins finally broke through in the sixth, though, to tie the game. Left fielder Marcell Ozuna had a two-out hit after a leadoff bunt single by catcher Jeff Mathias and right fielder Giancarlo Stanton delivered a broken-bat bloop double after earlier checking his swing on a 1-2 pitch.
The Braves had scored an unearned run in the fourth after a throwing error by Hechavarria and Eovaldi's wild pitch. Ryan Doumit, playing left field for Justin Upton, produced the RBI single.
Harang allowed a walk and a single to start the start the seventh inning, but Jordan Walden came in strike out Hechavarria and then pinch hitters Jarrod Saltalamacchia and Greg Dobbs.
"I think we won the game there in the seventh with Walden," Gonzalez said.
NOTES: Marlins LF Christian Yelich, who had struck out his first three at-bats, extended his hitting streak to 17 games with a leadoff bunt single in the eighth inning. ... Miami RHP Carlos Marmol left with a right hamstring strain after walking RF Jason Heyward, the third batter he faced in the seventh inning. ... The Marlins' 1-0 victory on Tuesday behind RHP Jose Fernandez was historic. It was the first time since 1900, according to the Elias Sports Bureau, that there had been as many as 28 strikeouts in a game without a walk. Fernandez fanned 14 in eight innings and LHP Alex Wood recorded 11 over eight for the Braves. ... Miami recalled RHP Carter Capps from Triple-A New Orleans on Wednesday to take the bullpen spot previously held by RHP Arquimedes Caminero, who was sent down after Tuesday's game. ... Both teams are off on Thursday. The Braves host Cincinnati over the weekend, while the Marlins go to New York to play the Mets.