Halladay (0-1), who struggled with a sore shoulder last season and had a difficult spring training, gave up five runs and six hits in 3 1/3 innings. The two-time Cy Young Award winner struck out nine, but dropped to 0-4 with an 8.00 ERA in his last seven starts against Atlanta.
Paul Maholm (1-0) allowed six hits in 5 2/3 scoreless innings. He struck out six and walked one.
Gattis led off the fourth inning of his first major league game with a drive to left.
The Phillies were just 1 for 11 with runners in scoring position.
In other National League games on Wednesday it was:
‧ Nationals 3, Marlins 0
‧ Mets 8, Padres 4
‧ Pirates 3, Cubs 0
‧ Rockies 7, Brewers 3
‧ Giants 5, Dodgers 3
‧ Diamondbacks 10, Cardinals 9
AP, Cincinnati, Ohio
In Cincinnati, Ohio, Brandon Phillips hit a three-run homer in his new role as the Cincinnati Reds’ cleanup hitter, and Joey Votto drove in the winning run with a ninth-inning single as the Reds beat the Los Angeles Angels 5-4.
Votto’s first hit of the season off left-hander Scott Downs (0-1) deflected off the glove of first baseman Albert Pujols and rolled into the outfield, allowing Choo Shin-soo to score from second base. Choo reached when he was hit by a pitch and advanced on a sacrifice.
Votto had been 0 for 6 heading into the decisive at-bat.
Phillips moved into the cleanup spot because of Ryan Ludwick’s injury and hit the Reds’ first homer of the season. He connected for a three-run shot against C.J. Wilson in the fourth inning.
Left-hander Aroldis Chapman (1-0) walked one in the ninth, throwing a 99mph pitch that hit the backstop screen.