Gallagher went 2-for-3 on the night, drawing a walk in the second and hitting a double in the ninth. But it was the grand slam off fellow rookie Jharel Cotton that helped the Royals pull to within a half-game of the second American League Wild Card spot.
"It was awesome," Royals rookie starter Jake Junis said. "So happy for him. It was a big point in the game, too, to get those runs."
The Royals led, 2-1, entering the sixth and loaded the bases with one out. After Alex Gordon popped out, Gallagher took three straight balls to open the count. He took a strike and then fouled off an up-and-in fastball.
"I could tell on his 3-1 swing, when he took a hack, he was ready," Junis said. "I think he got a similar pitch, and he handled it and put it out of the yard."
With all three runners on the move with the pitch, Gallagher connected on a 92.8-mph inside fastball and pulled down the left-field line 347 feet, according to Statcast™. Both Gallagher and Yost just prayed it wouldn't hook foul.
"I thought it was going to, but it stayed pretty true," Gallagher said. "Right off the bat I was like, 'Aah.' I kind of saw it in the air and I was like, 'It might have a chance.' It kind of did."
Yost added, "We needed to tack on there. We had bases loaded, Alex popped up, and I was like, 'Man, we could use a little breathing room.' … I was just hoping for one run, and when he hit the ball, I was hoping it would stay fair. It didn't hook much, it stayed pretty straight right down the line. Getting a five-run lead right there was pretty big."
Gallagher was called up Aug. 6 to replace All-Star catcher Salvador Perez, who was placed on the disabled list with a right intercostal strain. The 24-year-old made his MLB debut that day and picked up his first career hit, his only hit entering Monday night.
While Perez continues to work his way back, having Gallagher follow up catcher Drew Butera's four-hit day Sunday is big for the Royals.
"We've got pretty big production out of our catching spot the last two days," Yost said. "Salvy's a big bat. For these guys, my focus is just catch a winner. I don't care what you do offensively. It was definitely was a big night for him and was a big night for us."