Gallo hit a two-run shot into the Allegheny River in the third inning of the Rangers' 9-6 win at the Pittsburgh Pirates, becoming the first player in major league history to hit 100 home runs before reaching 100 singles (he has 93).
The blast, estimated at 443 feet, was Gallo's 100th career home run in his 377th game, making him the fastest player to 100 homers in American League history.
"Yeah, that's pretty crazy," Gallo said. "Like, if you would have told me that I would do that three years ago, I would think you were crazy. Pretty cool accomplishment, honestly. Be in the history books forever, so that's pretty special."
In setting the American League record, Gallo surpassed Mark McGwire, who hit his 100th homer in his 393rd game with the Oakland Athletics. Ryan Howard owns the major league record, recording his 100th in his 325th game with the Philadelphia Phillies.
Before Gallo hit his 100th, the player with the fewest singles in reaching that milestone was journeyman first baseman Russell Branyan, who had 172 singles when he hit his 100th home in 2004 while with the Milwaukee Brewers.
Gallo, 25, made his MLB debut in 2015 with the Rangers.
Wednesday's blast was his 12th homer this season.