OAKLAND, Calif. -- When Kansas City Royals rookie catcher Cam Gallagher got ahead 3-0 in the count with the bases loaded and two outs in the sixth inning Monday night, he was just looking for a walk.

Gallagher happily settled for a grand slam, his first major league home run, on a 3-2 pitch from rookie right-hander Jharel Cotton. The blast powered the Royals to their third consecutive win, a 6-2 victory against the Oakland Athletics at the Oakland Coliseum.

Gallagher homered in his 10th major league at-bat, his third game and his second start since being called up from Triple-A Omaha on Aug. 6. He was promoted when the Royals placed catcher Salvador Perez (right intercostal strain) on the disabled list.

"I know my timing hasn't been very good the last few days, the last few games, and my first two at-bats I was late," Gallagher said. "I just didn't feel right at the plate. When I got to two strikes, I kind of just shortened up, got my foot down and tried to put the barrel on the ball, and luckily it got the barrel and went over the fence."

Gallagher lined Cotton's fastball down the line into the left field seats, giving Kansas City a 6-1 lead. The Royals had loaded the bases on Melky Cabrera's leadoff walk, Mike Moustakas' sharp single to center and Alcides Escobar's one-out infield single.

"It was awesome," said Royals rookie right-hander Jake Junis, who pitched six strong innings for the win. "So happy for him. It was a big point in the game, too, to get those runs. Luckily the next inning I didn't get too much damage done and we got out of that and kept the pretty decent lead."

Gallagher, who entered the game 1-for-7 with no RBIs or extra-base hits, also hit a leadoff double to left-center in the ninth inning.

Junis (5-2) was called up Monday from Omaha and allowed two runs on four hits for his third straight victory, including one in relief. He struck out two, walked none and hit two batters. For the second consecutive time, he got a big-league victory while pitching to Gallagher, his teammate for five minor league seasons.

"He's a competitor," Gallagher said of Junis. "I know after we came in after that next inning where he gave up that run, he said he was so happy for me he had to lock it in. That's just the kind of relationship we have. He's a competitor and did a heck of a job tonight. He had all four pitches working and came out with a win."

The Royals (60-58) remained five games behind Cleveland in the American League Central and moved to within a half-game of the Los Angles Angels for the second AL wild-card spot. They had lost five straight games and nine of 11 before going on their three-game streak.

"I feel like we're back on track," Royals manager Ned Yost said.

The A's (52-67) cut Kansas City's lead to 6-2 in the bottom of the sixth. Boog Powell lined a leadoff single and moved to third on Matt Joyce's single. Junis hit Ryon Healy with a pitch to load the bases with one out, then hit Khris Davis, forcing in a run. Junis retired Matt Olson on a foul popup and struck out Chad Pinder to end the inning.

"We just had a rough game all the way around, and when we had opportunities we didn't come through with them, and when we had a chance to get out of innings, we didn't get out of them," A's manager Bob Melvin said.

Marcus Semien went 2-for-4 with a triple and scored a run for Oakland.

Cotton (5-10) gave up six runs on eight hits over six innings. He walked three, struck out two and lost his third straight decision. He held the Royals to two runs until Gallagher's slam.

"I wasn't trying to be too fine," Cotton said. "I was just trying to give it my best on that pitch, and it just wasn't there."

Mike Minor, Brandon Maurer and Ryan Buchter each pitched a scoreless inning in relief for the Royals.

Kansas City took a 1-0 lead in the top of the first inning when Whit Merrifield hit a leadoff single, stole second base and scored on Eric Hosmer's single.

The A's pulled even with a run in the bottom of the inning. Semien tripled off the left-center-field wall with one out and scored on Healy's groundout.

Kansas City regained the lead at 2-1 in second inning. Escobar lined a two-out single to left, moved to third on Alex Gordon's single to right and scored on wild pitch by Cotton.

NOTES: Royals C Salvador Perez (right intercostal strain) caught a bullpen session and took 10 to 15 swings off a batting tee at 30 to 40 percent effort on Monday as he continued his recovery. Perez has been out since Aug. 5. "I feel pretty good," Perez said of swinging the bat. ... RHP Kevin McCarthy was optioned to Triple-A Omaha on Monday, opening a spot on the Royals' 25-man roster for RHP Jake Junis, who was called up and started against Oakland. ... A's INF/DH Ryon Healy hit safely in his eighth straight game against Kansas City to start his career. He went 1-for-3 with a double and an RBI and was hit by a pitch. ... Oakland RHP Bobby Wahl had successful thoracic outlet surgery in Dallas, the A's announced. He will return to Oakland on Thursday to begin his rehab.