The Brewers right-hander gave up just two hits in eight sparkling innings, throwing 110 pitches, as he struck out six Braves to post the fifth win in his last six decisions.
"It felt good out there," Gallardo told reporters. "For the first couple of innings, my command was a little off because I was overthrowing a little bit. As the game went on, I was able to correct those things and keep the hitters off balance.
Reliever Carlos Villanueva hurled a perfect ninth to seal victory.
Atlanta never threatened to score and new signing McLouth finished 0-for-4. The center-fielder, acquired from the Pittsburgh Pirates on Wednesday, was an All-Star last season and is expected to bring added power to the Braves' lineup.
Facing Gallardo, however, the Braves (26-27) failed to advance a runner past second base and dropped their seventh game in the last 10 contests.
The National League Central-leading Brewers (32-23) scored two runs apiece in the fourth and fifth innings to give Gallardo a comfortable cushion. Rookie Mat Gamel put Milwaukee on the board with a two-RBI double and finished 2-for-4 with three RBIs.
"What got me here (is) my ability to hit the ball the other way," Gamel said.
"I've been trying to not do too much. You can't go up there trying to do too much. It's a game based on failure as it is. You can't put too much pressure on yourself."
Brewers first baseman Prince Fielder stayed hot with two hits and one run. Fielder is 13-for-29 in his last eight games.
Atlanta ace Jair Jurrjens lost for the first time in four starts despite eight strikeouts, allowing four runs in seven innings.
