After the Giants cruised to a blowout win in the first game of Tuesday’s doubleheader against the Rockies, manager Gabe Kapler said he was glad that his club never took its foot off the gas pedal, noting that no lead ever feels safe at Coors Field.

The Giants learned that lesson the hard way in a gut-wrenching 8-6 walk-off loss in their nightcap in Denver.

• Box score

San Francisco took a 6-2 lead into the bottom of the seventh inning, but the Rockies staged a six-run rally that was capped by Charlie Blackmon’s three-run, game-winning homer, securing a split of the twin bill between the National League West foes.

The 18-12 Giants retained their perch atop the division, but they missed a chance to create some separation from the Dodgers and Padres, who are only 1 1/2 games out of first place.

• Giants cruise behind 10-run 1st inning

Closer Jake McGee, who hadn’t pitched in a week, opened the bottom of the seventh and allowed a two-run double to Trevor Story, followed by an RBI single to Ryan McMahon that trimmed the Giants’ lead to 6-5.

Kapler then summoned rookie Camilo Doval to face C.J. Cron with a runner on first and two outs, but Cron blooped a single to right field to put runners on the corners for Blackmon.

Blackmon worked a 3-2 count before crushing a hanging slider from Doval out to right field for the decisive blast.

Despite the disappointing outcome, Kapler opened his postgame Zoom session with reporters by praising the performance of the 23-year-old Doval, who has been thrust into several high-leverage situations in his first eight games in the big leagues.

“I don’t want to miss the fact that, of course, he ended up getting beat on a slider to one of the better hitters in the division,” Kapler said of Doval, the Giants' No. 24 prospect per MLB Pipeline. “But I was really proud of the work that he did. We asked him to come in, throw strikes, control the running game, stay composed, execute two pitches. That’s all he did.

“Obviously, that’s a very difficult loss to swallow. In particular, you’re going to have a young man that is feeling like he’s responsible for it. And he’s not responsible for that loss. He did everything that we could possibly ask of him.”

There is more concern about McGee, who did not allow a run in his first eight appearances but has now been charged with eight runs in his last six games, causing his ERA to spike to 5.54 on the season.

“His fastball doesn’t have its best life right now,” Kapler said. “When he’s at his best, his fastball is really carrying and he’s missing more bats than he is right now. That’s something that we’re going to continue to work on with him and support him through. Obviously, he’s not at his best, and I think that’s important to acknowledge.”

Left-hander Alex Wood, who played with McGee on the Dodgers in 2020, expressed confidence that the 34-year-old veteran would bounce back quickly from the rough outing.

“Jake’s a total pro,” Wood said. “He’s been doing it for as long as anybody on our team. I think everybody in our clubhouse has total confidence in him. Over the course of a 162-game season, you hit little runways like this where you might struggle a little bit, but he’s been doing this for a long time. He’s got the confidence of me and everybody in our clubhouse, our coaching staff. I’m sure he’s going to be right back out there tomorrow, hopefully to lock down the game.”

The bullpen implosion overshadowed a huge day for first baseman Brandon Belt, who achieved a cool bit of history on Tuesday.

After launching a first-inning grand slam in the first game, Belt stuck to a similar script in the nightcap, crushing a two-run homer off Ryan Castellani to put the Giants on the board in the first inning.

According to the Elias Sports Bureau, Belt became only the second Giants player in the San Francisco era to homer in the first inning of multiple games on the same day, joining Hall of Famer Willie Mays, who accomplished the feat on May 22, 1965, against the Astros.

“Obviously, that’s pretty cool,” Belt said. “I’ve never really thought about that before, but to be mentioned in the same sentence as Mays, that’s pretty awesome.”

Belt, 33, went 1-for-12 with seven strikeouts over the Giants’ three-game series in San Diego this past weekend and entered Tuesday hitting .203 in 27 games this year. He raised his batting average by 32 points after going 4-for-7 with seven RBIs across both ends of the doubleheader.

“I’ve been working pretty hard to get back on track,” Belt said. “To have a good day after that series I had in San Diego, it feels pretty good. Obviously, it would have been better if we had pulled off two wins today, but we can make it all right by winning tomorrow.”

Belt has shown the ability to carry the offense for long stretches when he heats up, and the Giants will need consistent production from him as they attempt to withstand injuries to three of their top hitters: Mike Yastrzemski, Donovan Solano and Tommy La Stella, who landed on the injured list with a left hamstring strain on Tuesday.

“His at-bats are professional, they're consistent,” Kapler said of Belt. “Sometimes he gets rewarded for hard contact and sometimes he doesn't. But we have a ton of confidence in Brandon Belt and his ability to produce over the long haul. Every time he steps in the batter's box, I feel like something good is going to happen.”