The D-backs took themselves out of the running for a win early when the Giants pounded Patrick Corbin for six runs in the second inning.
There were few warning signs a big inning was coming as Corbin retired the Giants in order in the first.
Buster Posey led off the second with a single to center and Hunter Pence followed with a homer. After Hector Sanchez drove in a run with a double and scored on a passed ball, Marco Scutaro capped the inning with a two-run homer.
"Just didn't make good pitches," Gibson said. "They were whopping him early in the count. He gave up six runs on 19 pitches, two home runs. A couple he just tried to come in and left them out over the plate."
Corbin was making just his 16th Major League start, and part of the learning process for a young pitcher is figuring out how to stop an inning from getting out of control like that.
"I just have to learn to minimize damage," Corbin said. "I just made a couple of bad pitches, the first pitch to Pence that he hit out and left the change up to Scutaro that he hit out. I've just got to make better pitches. It just kind of unraveled a little bit. I've just got to go out there and make better pitches on those counts instead of just throwing the fastball in there and letting them hit it."
Corbin (6-8) allowed six runs on seven hits over three innings. He has one more regular-season start next Tuesday against the Rockies.
"I feel fine; it's just I'm not executing right now like I should be," Corbin said. "I'm just trying to go out there and learn from my mistakes. I'm definitely looking forward to my next start to just go out there and end on a good note."
It was a much different start at the plate for the D-backs, who got a triple to lead off the game from A.J. Pollock and a one-out double from Jason Kubel in the second, but stranded them both.
"They came out swinging the bats and we didn't start good," Gibson said. "In our first two innings, we were 0-for-5 with runners in scoring position and they came back and scored six runs on us."
That was more than enough run support for Barry Zito (14-8). The Giants left-hander lasted six innings to raise his record against the D-backs this year to 4-0.
Zito kept the D-backs at bay until the fourth when, with two outs and a runner at second, he lost his feel for the strike zone and issued back-to-back walks to load the bases.
Pinch-hitter Chris Young then blooped a single to left to score a pair and cut the San Francisco lead to 6-2.
"I don't know how to explain it, honestly," Young said when asked about Zito's dominance of the D-backs this year. "His [velocity] is down obviously from where it used to be, but he's gotten a lot smarter as a pitcher, I think. He has a little late movement and tries to keep guys off balance with a big slow curveball that makes his fastball play up a little bit. I think in guys' heads you think that he's throwing so slow that you actually wait too long and end up getting jammed. He's been staying off our barrels real good and we haven't been able to make the adjustment."