Even Nick Punto, who entered Sunday 4-for-42 over his previous 15 games, added a solo home run in the ninth to give the A's some extra insurance.
But the afternoon belonged to Gray (10-3). The 24-year-old Tennessean struck out five, walked two and allowed six hits en route to his fourth win in his last five starts while improving to 7-1 this season against the American League West.
"He really wanted to go out there and give us that type of effort today so we could have a good feeling going into this break," manager Bob Melvin said. "You never want to get swept the last three games going into the break, it just doesn't feel very good."
At 59-36, the A's enter the All-Star break with the best record in the Major Leagues. This is the second year in a row the club enters the break in first place in the AL West.
On Sunday, the offense snapped out of a two-day lull that was the product of injuries and of facing the Mariners' 1-2 punch of Felix Hernandez and Hisashi Iwakuma.
After combining to score four runs off Hernandez and Iwakuma the past two days, Oakland finally broke through -- even though regulars Derek Norris (rest), Coco Crisp (sore neck), Josh Reddick (strained right knee) and Alberto Callaspo (strained right hamstring) were out of the starting lineup.
The A's had just one hit -- an Andy Parrino double -- off Mariners starter Chris Young (8-6) through four innings, but they used a pair of RBI singles from Craig Gentry and John Jaso in the fifth to grab a 2-1 lead. Then, with one out in the sixth, Moss launched a towering home run into Safeco Field's right-field seats to give Oakland the 3-1 advantage.
"It was just a fastball middle-in," Moss said after driving in his 66th run of the season. "It was 2-0, I had a good count, and I was just looking for a pitch right in that zone."
Gray's only blemish came on an unearned run in the first inning after an errant pickoff attempt went wide of first base and into right field, allowing James Jones to advance all the way to third base. Robinson Cano followed with a dribbler back to the mound that allowed Jones to score.
In the fourth inning, Cano and Kyle Seager led off with back-to-back singles to put runners on first and third. Gray countered by inducing Logan Morrison to hit into a weak fielder's choice before getting Justin Smoak to pop out to Josh Donaldson. Gray then struck out Dustin Ackley to wiggle out of trouble.
"I was trying to throw my best fastball, my best curveball, my best changeup in the perfect spots," Gray said after improving to 3-0 with a 0.87 ERA over 20 2/3 innings against Seattle this season. "If I would have walked one of them, then so be it, but I was nibbling in my strength spots hoping to get soft contact, and I was able to do that."
He continued to roll until giving way to closer Sean Doolittle, who recorded the final four outs for his 14th save of the season, thus helping the A's regain a bit of momentum going into the break. "We've had a great first half, we've played really good baseball," Moss said. "I think there's not a team in the league that wouldn't want to be where we are … but at the same time there's 2 1/2 months left and a lot of games against our division, so we have to continue to play good baseball because our division is playing really well."