Gallardo (8-2) pitched six effective innings to win his sixth consecutive start and Milwaukee posted back-to-back wins in South Florida for the first time since April 10-11, 2007. The Brewers began the day with the fewest road wins in the National League, but improved to 11-19 away from Miller Park.
"The players know that we're playing well now, and I know that we're playing well now," Brewers manager Ron Roenicke said. "I know you guys still want to ask us about the road but I think that's probably more you guys than it is us."
Gallardo has a 1.32 ERA during his winning streak, which started with a victory at St. Louis on May 7. The right-hander improved to 3-0 with a 1.72 ERA in his career against Florida.
"I'm throwing the ball pretty well right now," Gallardo said. "I'll enjoy the win today but keep working."
Florida left nine runners on base and went 1 for 10 with runners in scoring position. A night earlier, Florida stranded 11 runners in another one-run loss.
"We're not executing," Marlins manager Edwin Rodriguez said. "That wasn't only last night or tonight. We'be been watching that for two months."
The Brewers trailed 2-1 before Fielder hit his 13th homer with one out in the seventh. The burly first baseman sent Chris Volstad's 0-1 pitch into the seats in left-center.
Volstad (2-5) gave up six hits, struck out four and walked one in seven innings. He dropped to 0-4 with a 5.87 ERA in his last seven starts.
Gaby Sanchez hit a two-out RBI single in the first and the Marlins pushed across an unearned run in the sixth to take a 2-1 lead.
Sanchez opened with a single and took second on Greg Dobbs' grounder to first. Following Mike Stanton's groundout and a walk to John Buck, Omar Infante hit a grounder to third. McGehee had trouble backhanding the ball, then threw high to first for an error, allowing Infante to reach and Sanchez to score.
Nyjer Morgan hit Volstad's first pitch of the third into the right-field seats to tie it at 1. It was Morgan's first homer of the season.
Florida failed to score despite putting runners on second and third with none out in the third. Gallardo escaped by retiring Bonifacio, Logan Morrison and Sanchez.
"That was a huge inning for me, and it was for them. You can ask the other side," Gallardo said. "Getting one, two, three in the heart of the lineup is big."
Marlins RHP Josh Johnson (right shoulder inflammation) long tossed in the outfield for a second straight day. Manager Edwin Rodriguez said Johnson had no pain but added there were no scheduled plans for the ace to throw off the mound. Johnson has been on the disabled list since May 21. ... Marlins SS Hanley Ramirez (stiff lower back) missed his fifth game in a row. Rodriguez said the team probably will wait until Monday to decide whether to put Ramirez on the DL. .... While playing for Washington last year, Morgan charged the mound after Volstad threw a pitch behind him. Morgan had run over Marlins C Brett Hayes a night earlier, leaving the catcher with a separated shoulder. ... Fielder has a 10-game hitting streak at Florida.