"It feels a lot better. Just the positivity around the clubhouse, the ambience is a lot better," said Fielder, who was often mentioned in trade rumors last year with the Brewers on their way to a second straight losing season. "It's just a good feeling knowing everybody's here to win and everybody's rooting for each other."
Fielder's drive in the seventh was the second-longest in Miller Park's 10-year history and a punctuation mark for a Brewers offense that had struggled over the last 10 days.
"Corey started it off with the solo homer, that got the juices flowing a little bit and Yo pitched great, shutting them down that whole time," Fielder said. "Whenever your starting pitcher is doing that, you tend to relax."
Gallardo (12-7) allowed an unearned run and four hits, retiring 15 of 16 at one point. He also struck out seven and walked none while improving to 6-0 with a 3.50 ERA in his last six starts against Houston dating to 2009.
Strong starting pitching has been the key to Milwaukee's recent run. Since Gallardo had an ugly outing in Colorado the first game after the All-Star break, the Brewers' starting rotation has allowed three runs or less in 15 straight starts.
"We all see what the other guy in front of us is doing," Gallardo said. "You've just got to try to match that or try to make it better. It's going to help the team overall."
Houston was well on its way to one of the worst seasons in franchise history even before trading Hunter Pence to the Phillies for three minor leaguers and a player to be named during Friday night's game.
The Astros scraped together four singles off Gallardo, including one by Michael Bourn, who also is the subject of trade rumors in the final hours before Sunday's non-waiver deadline.
Ryan Braun started the inning with a grounder to shortstop Angel Sanchez, who bobbled it for an error. Fielder singled and Casey McGehee followed with a sacrifice fly. After a single by Yuniesky Betancourt, Lucroy hit a two-run double to make it 5-0.
Happ (4-13), who was acquired last year from Philadelphia in the Roy Oswalt deal, gave up five runs, two earned, and 10 hits in five innings. The left-hander went 1-4 with a 7.48 ERA in five starts in July.
The Astros scored an unearned run off Gallardo on Sanchez's RBI single in the sixth. Pinch hitter J.D. Martinez doubled in another run in the eighth in his first major league at-bat after taking the roster spot of Pence.
"It was sure nice to see J.D. put a charge into that ball and get his first major league hit," Astros manager Brad Mills said. "That was sure nice."
Fielder's 24th homer came off Aneury Rodriguez and landed beyond the right-field bleachers, hitting a stadium support below the Brewers' clock to give Milwaukee a 6-1 lead and its biggest offensive output since scoring 11 runs in a win at Arizona on July 19.
Fielder said all of his home runs count as one, but Hart said it's hard not to have an awestruck look whenever Fielder hits one that far.
"Usually when you have one everybody gets excited. When they're hit that far, you don't know what to do," Hart said. "He's a strong individual."
Russell Branyan owns the Miller Park record with a 480-foot blast in 2004. ... The Brewers traded a minor leaguer to Washington for utilityman Jerry Hairston Jr., who arrived at the ballpark shortly after the game began and was seen in the dugout in the second inning. ... The Astros assigned 1B-OF Jonathan Singleton to Class-A Lancaster and RHPs Jarred Cosart and Josh Zeid to Double-A Corpus Christi. The three were part of the Phillies' package for Pence. ... It was the 16th sellout at Miller Park this season with 44,306 in attendance.