The Reds' No. 4 prospect was 3-for-5 with a walk-off homer as Triple-A Louisville bested Buffalo, 5-4, on Wednesday.
After the Bats rallied to tie the game in the eighth inning, Gregorius took the third pitch of his at-bat against Bisons reliever Pedro Beato over the right-field wall to deliver the win for the Bats.
"I was just happy," Gregorius, who scored twice, told the Louisville Bats. "I wasn't looking for any particular pitch. I hit a fastball, I put a good swing on it. As a hitter, you always look for the fastball. If you get a different pitch, you have to adjust. But I got a good pitch I could handle."
Since being promoted to the International League on July 12, the 22-year-old shortstop is batting .321 with four homers and nine RBIs in 14 games. To put the power numbers in perspective, Gregorius slugged a career-high seven homers in 84 Minor League games in 2011, after having one longball in 81 games for Double-A Pensacola before his promotion.
"They say you have to have power to hit home runs, but for me, if I make a good swing and the pitcher throws hard, the ball can go," Gregorius said. "For me, it's just making contact."
Gregorius has a five-game hitting streak with two hits in each of his last three contests for the Bats.
Trailing by four entering the eighth, the Bats got RBI singles from Dioner Navarro and Felix Perez before Cody Puckett tied the game with a two-run single.
J.J. Hoover allowed one hit and struck out two in one inning to improve to 4-0 for Louisville.
Collin McHugh allowed five hits and struck out six over 6 2/3 shutout innings but did not figure into the decision for the Bisons.