ATLANTA -- New York right-hander Robert Gsellman pitched 6 2/3 scoreless innings and left fielder Yoenis Cespedes hit a grand slam in his return from the disabled list as the Mets beat the Atlanta Braves 6-1 in the first game of a day-night doubleheader at SunTrust Park.

Gsellman (5-3) allowed only three hits, with two walks and four strikeouts. It was the first time in 11 starts this season that he has not allowed a run.

In four starts since returning from a stint in the bullpen, Gsellman has allowed seven runs, six earned, in 25 innings.

Cespedes, out since April 28 because of a strained left hamstring, was 2-for-5 with a home run, his seventh. Cespedes belted his fifth career grand slam in the eighth inning off reliever Luke Jackson to put the game away.

Atlanta left-hander Sean Newcomb (0-1) made a successful major-league debut. He pitched 6 1/3 innings and allowed one unearned run on four hits and two walks, with seven strikeouts. His throwing error led to the run.

Newcomb, the No. 15 overall pick by the Los Angeles Angels in the 2014 draft, was acquired in a trade that sent shortstop Andrelton Simmons to the Angels prior to the 2016 season. He became the first Atlanta player to make his major-league debut at SunTrust Park.

The Mets scored their first run in the second inning. Wilmer Flores led off with a single and Asdrubal Cabrera hit a comebacker to Newcomb, which appeared to be an ideal double-play ball.

However, Newcomb's throw to second base was in the dirt, rolled into the outfield and allowed Flores to reach third. T.J. Rivera drove him in with a sacrifice fly to center field.

New York added a run in the eighth against Jackson to take a 2-0 lead. Michael Conforto was hit by a pitch and came around to score when Flores struck a sacrifice fly to center field.

Atlanta scored in the eighth when Brandon Phillips led off with a solo homer to left, his fourth, off Fernando Salas. The Braves put runners on first and second base when Addison Reed entered the game.

Reed struck out pinch-hitter Matt Kemp and got Rio Ruiz to ground out and finished the game with a scoreless ninth.

NOTES: The Mets activated LF Yoenis Cespedes from the disabled list. He had been sidelined since April 28 because of a left hamstring strain. Cespedes was 0-for-4 in a rehab start for Class A Port St. Lucie on Thursday night. ... New York will activate LHP Steven Matz for the second game of the doubleheader and will make a roster move to clear a space for him at that time. Matz has been on the disabled list all season because of left elbow inflammation. He was 0-1 with a 6.06 ERA in four rehab appearances. ... Atlanta sent RHP Jason Hursh to Triple-A Gwinnett to make room for LHP Sean Newcomb, who made his major-league debut in the first game.