The Royals also called up catcher Tony Cruz, and selected the contract of third baseman Hunter Dozier.
Reliever Luke Hochevar was transferred to the 60-day disabled list to make room on the 40-man roster for Dozier, the Royals' first-round pick in the 2013 Draft.
Gore and Merrifield join a track-caliber roster already possessing sprinters Jarrod Dyson, Paulo Orlando, Raul Mondesi, Billy Burns and Lorenzo Cain.
"Never been a part of a team with this much speed," manager Ned Yost said. "Never."
Yost now will have even more late-inning pinch-running options.
"We're not a team that is going to mash 15 runs a game," Yost said. "But if we can keep it close, we can manufacture runs with that type of speed."
Those pinch-running options are one reason the team promoted Dozier.
"If I pinch-run for Cheslor [Cuthbert], we can have an experienced third baseman to replace him," Yost said.
Merrifield, a super utility player, will float from second base, to shortstop, to third base to all outfield positions, when needed.
Merrifield is making his second stint with the Royals this season. He hit .271 in 53 games prior to Friday's series opener with the Tigers.
Davis had been sidelined since July 31 with a right flexor strain. Davis did not go back-to-back games during his injury rehab stint, and Yost will use caution with Davis early on.
"We'll just see how it goes with him," Yost said. "Take it day to day."
The Royals likely will make more callups next week. Infielder Christian Colon, who was sent out on Wednesday, will return after Triple-A Omaha's season ends on Monday.