Gamel, who replaced Fielder at first for Milwaukee this year, scored the winning run on a close play at home on a double by pinch-hitter George Kottaras with one out in the bottom of the ninth inning, lifting the Brewers to a 5-4 victory over the Los Angeles Dodgers on Tuesday night.
"I knew it was going to be close," Gamel said. "I figured I could get around (catcher A.J. Ellis) just because of where he was on the plate. I was going for it."
The Brewers snapped a four-game losing streak and ended the Dodgers' six-game winning streak.
"Definitely the best game I've had at this level," said Gamel, who also hit his first home run since 2009.
Gamel was one of the franchise's top prospects late in the last decade, but stalled for a couple years in Triple-A. When Fielder signed with the Detroit during the offseason, Gamel was given the job at first base.
He has held his own so far, batting .282 while starting all 11 games for Milwaukee. He said he has made some minor mechanical adjustments in the last couple of days that have helped.
"I finally hit the ball the way I know I'm capable of," said Gamel, who finished 2 for 3 with a home run, a single, a walk and a stolen base that led to a run in the seventh.
Guerra, who blew his first save of the year and third in his career, struck out Jonathan Lucroy. Kottaras then batted for Cesar Izturis and hit a 2-2 pitch to the wall, driving in Gomez and Gamel with Gamel sliding in just ahead of the throw from second baseman Mark Ellis and sending the Brewers to a jubilant celebration at home plate.
It was the first blown save of the year for Guerra, and only the third of his career.
"I will own up to this one," Guerra said. "It was my bad. For the ball club to have battled back like we did, those are the games we have to win."
Kottaras, who starts every five days as Randy Wolf's personal catcher, is 2 for 5 with a double, a home run and five RBIs as a pinch-hitter this year.
"He threw me a slider, like a cutter-slider, and I just covered it well and hit that double," he said.
Jose Veras (1-0) pitched a scoreless ninth to earn the victory.
Milwaukee was 57-24 at Miller Park last year, and Brewers manager Ron Roenicke said his team is comfortable in the stadium.
"We are confident we can come back and win ballgames," he said. "That does mean a lot when you get late in the game and you're facing a guy whose coming out of that bullpen and really throwing well."
"You don't feel real good about your chances when you get late in the games against great relievers," he said. "We all feel like we can come back and win."
Andre Ethier's two-run home run with two outs in the eighth inning had given the Dodgers their 4-3 lead. Mark Ellis hit a one-out broken-bat single off Francisco Rodriguez and, after Matt Kemp struck out, Ethier hit a 1-1 pitch into the Brewers' bullpen for his fourth homer of the season.
The game featured National League MVP Ryan Braun against the runner-up in Kemp. Braun went 0 for 3 while Kemp was 1 for 4 with a double.
Milwaukee starter Yovani Gallardo gave up one run and two hits in seven innings. He had one intentional walk and struck out seven.
The right-hander was rocked for six runs in just 3 2/3 innings on opening day against the Cardinals, but has given up just three runs over 14 innings (1.93 ERA) in his last two starts.
Los Angeles starter Chad Billingsley gave up two runs and five hits with six strikeouts in six innings. He has walked only one batter over 20 1/3 innings in three starts.
Brewers radio announcer Bob Uecker missed the game to be inducted into the National Association of Broadcasters Hall of Fame in Las Vegas. ... Brewers SS Alex Gonzalez missed his fourth consecutive game due to the birth of his son, Axel David. He is expected to return for Wednesday's game. ... Gamel's home run was his first since Sept. 27, 2009, in a 6-5 loss to Philadelphia.