Shortstop Bobby Crosby also misjudged a soft liner by Ichiro Suzuki that extended the third-inning surge. Second baseman Cliff Pennington got caught going one way when a sharp single by Yuniesky Betancourt went another.
Gallagher also missed his spots on a couple of fastballs that Jose Lopez hit for a pair of home runs and four RBIs.
"Baseball is a game of inches, it really is," said Gallagher, now 5-7. "Sometimes a pitch is an inch here and an inch there. Fielding plays. It's everything. Something it goes your way, sometimes it doesn't."
A's manager Bob Geren echoed that, adding, "He had a few pitches up and he had a few strange breaks not go his way.''
The A's had jumped ahead, 2-0, in the second. Jack Cust opened with a long home run to right, his fourth in five games. Jeff Baisley later had a two-out, RBI single.
The Mariners then batted around in the third to score their first five -- all with two outs.
With one out, Luis Valbuena looped a ball to shallow left. Buck came sliding in for it, but it got past him for a double. Geren said that was probably one play that Buck should have been more cautious with. It opened the door to home for Seattle.
Ichiro and Raul Ibanez followed with RBI singles. Lopez finished it with his three-run home run into the A's bullpen in left.
After Buck's third-inning adventure, he made up for it with his bat, driving in four runs, three of them on a fourth-inning home run. He is 13 of 39 (.333) with three home runs and 10 RBIs in 10 games since being recalled from Sacramento Sept. 16.
"It's always been in him," Geren said. "He's getting to balls on both sides of the plate. He's pulling some balls a long way and he hit some balls the other way well."
The A's then went ahead, 6-5, on Carlos Gonzalez's run-scoring single in the fifth.
But Gallagher couldn't hold it. The Mariners pushed five more runs across in the fifth, three of which were charged to Gallagher. The young right-hander, who is 5-0 with a 3.41 ERA in home games, is now 0-7 with a 6.91 ERA in his career on the road.
"Every day is different," Gallagher said. "I'm not worried about notching a win. I just want to keep my team in the game, get the whole team a chance to get a 'W,' not just me.
"I prepare the same way at home. I prepare the same way on the road. I don't do anything different. Weird thing."
It was weird all around for him.
But Gio Gonzalez had a quietly normal outing. He entered in the sixth inning and shut down the Mariners offense. He worked three innings, allowing just two hits, no runs, walking one and striking out four. He struck out the side in the eighth, Ichiro, Betancourt and Ibanez.
"I tried to put the scoreboard at 0-0," Gonzalez said. "I was focused in the bullpen. I blocked everything out. The whole time I thinking, 'Let's get this team out and my team back in there to score some runs.'
"This is the way you want to finish. You want to leave it in the back of their heads, 'He can do this and he can continue doing this.' I want to leave on a good note."
Geren said that Gonzalez is playing a happy tune.
"He allowed a bloop double and one other single and that was it," Geren said. "The way he finished, striking out the 1-2-3 hitters was impressive. He had a sharp curveball, good velocity. His fastball was tailing into lefties. It was a good outing."
The A's added a run in the ninth on Kurt Suzuki's RBI single, scoring Ryan Sweeney from second.
"We scored eight runs," Geren added. "You score eight runs in this ballpark, you should win."