Gallo attended Bishop Gorman High School in Las Vegas, Nevada.  As a senior, he was the Nevada Gatorade Baseball Player of the Year and a High School All-American by MaxPreps, ESPN HS and USA Today.     He hit a Nevada high school record 67 career home runs, and during his senior year of high school, he had a .509 batting average with 21 home runs and 80 RBIs. 
Gallo started his professional career with the Arizona League Rangers, where he hit .293/.435/.733 with 18 home runs and 52 runs batted in in 43 games. The 18 home runs were an Arizona League record.  He was named the Arizona League MVP.   He was promoted to the Spokane Indians of the Class A Short-Season Northwest League, hitting .214/.343/.464 with four home runs in 16 games.
Prior to the 2013 season, Gallo was the Rangers 10th best prospect according to Baseball America.  He played the season with the Hickory Crawdads of the Class A South Atlantic League. He became the first teenager since Dick Simpson in 1962 to hit 40 home runs in a minor league season.  Gallo won the Joe Bauman Home Run Award for hitting the most home runs in minor league baseball for the 2013 season.  Over the offseason, he worked out with Troy Tulowitzki and Jason Giambi at the Philippi Sports Institute in Las Vegas.  He started the 2014 season with the Myrtle Beach Pelicans of the Class A-Advanced Carolina League and was promoted to the Frisco RoughRiders of the Class AA Texas League in June.  In July, he played in the All-Star Futures Game, where he was named the MVP of the game after hitting a go-ahead home run in the sixth inning. 
Gallo began the 2015 season with Frisco. On June 1, the Rangers promoted Gallo to the major leagues.  In his first major league game, June 2, Gallo hit his first major league home run and drove in 4 runs (becoming the first Ranger to achieve four RBIs in an MLB debut game), going 3-4 in the game. He also had a single and a double in the game.  On June 5, 2015, Gallo earned his first MLB Golden Sombrero against the Kansas City Royals, making him the earliest to accomplish this in a Ranger career. 
On June 30, 2015, Gallo was optioned to Triple A to make room for Josh Hamilton. On May 12, 2017, Gallo hit his first career walk-off hit, a three-run home run off of Oakland Athletics' closer Santiago Casilla to complete a four run rally in the 9th inning. It was the second consecutive night the Texas Rangers walked off.