Garland (11-11) gave up an unearned run and seven hits in eight innings, tying his longest outing of the season. The right-hander is unbeaten in his last six starts, four of those with the Dodgers after he was acquired from Arizona on Aug. 31.
"It's nice to pitch a game and not have to worry," he said. "I'm starting to be part of this team a little more each day. They put in so much work since the beginning of spring training. Then to get new players in September, I've been on the other side of that and it's weird."
Garland is 3-0 with a 2.33 ERA in his four starts with the Dodgers.
"He had command of all his pitches and was working off the edges," catcher Russell Martin said. "He really didn't make any mistakes."
The Dodgers, who lost the series opener 8-4 on Friday night, began the day with a five-game lead over Colorado in the NL West. The Giants were 2½ back of the wild card-leading Rockies before Saturday's games.
The Dodgers' victory reduced their magic number to six for clinching their third postseason berth in four years.
"We got Jon a lead and he knew what to do with it. That was vintage stuff when you see a veteran pitcher with a lead and making them hit the ball," manager Joe Torre said. "It was nice to bounce back."
Loney went 3 for 4 and drove in two runs as the Dodgers improved to 10-7 against their rivals, with their largest margin of victory this season against them.
Belliard has hit four homers since joining the Dodgers on Aug. 30 from Washington.
"I was just looking for a pitch to hit. He gave me a fastball inside and I reacted to it," he said. "Garland was throwing strikes all day long."
Penny (10-9) defeated the Dodgers 7-2 last Sunday in his first start against them since being signed by the Giants on Aug. 31. The right-hander had been particularly tough on hitters with runners in scoring position and two outs this season, holding them to a .176 average.
But Los Angeles pummeled him with two outs in the first. Loney's RBI single tied it at 1 before Belliard sent a 1-2 pitch into the Dodgers' bullpen for his fourth career grand slam, giving them a 5-1 lead and triggering taunts of Penny. He spent five years in Los Angeles before being traded to Boston last year.
"It was pretty obvious he was off today with his command and his secondary pitches," Giants manager Bruce Bochy said. "He's been as good as you can be the first three starts and he had an off day. He was a pitch away from getting out of the first, but he made a mistake. It's going to happen."
Pitching in 87-degree heat, Penny gave up seven runs and five hits in 2 2/3 innings -- equaling his shortest outing of the season. He lost for the first time in four outings with the Giants.
"Today was on me. You just can't get down seven runs in a situation like this in September," he said. "I just wasn't making pitches. My focus wasn't there. The off-speed pitch I tried to locate on the grand slam was horrible. It was a changeup that went way up and in. If it's down and away, there's no way he hits it. With two strikes, you've got to bear down and make better pitches, and I didn't do that."
Randy Johnson made his first relief appearance since pitching for the Yankees in the 2005 American League division series. He faced three batters in the sixth, giving up consecutive doubles to Rafael Furcal and Andre Ethier before retiring pinch-hitter Juan Pierre.
Johnson was activated from the 60-day DL on Wednesday. He didn't stick around to speak afterward.
"It probably had to be strange for him," Giants pitching coach Dave Righetti said. "Once you go to the pen, the toughest thing is knowing when you're going to pitch in a race like this. To get him out there was important. Obviously, he's down, stuff-wise, but he's reporting no pain. But his arm strength is not there. So we have to make a decision on when and how to pitch him, in terms of rest."
Bochy said three batters was Johnson's limit going in.
"I knew he'd be rusty. It's been a while for him," he said. "The problem was just location more than anything and that's going to happen when you haven't been out on the mound in a long time. I'm sure he was a little hyped up. But it's good to see him out there."
The Dodgers extended their lead to 7-1 in the third. Kemp and Loney homered on consecutive pitches from Penny, with Loney clearing the right-field wall for his first at home after 12 on the road. Kemp connected on a 2-1 pitch for his 25th homer and 95th RBI -- both career highs.
Pinch-hitter Blake DeWitt also homered leading off the seventh for the Dodgers.
- Johnson's last regular-season relief outing came on July 18, 2001, at San Diego, when he struck out 16 in completion of a suspended game.
- Belliard's last grand slam was on Aug. 22 against Milwaukee.