The Texas Rangers selected Gallo in the first round, with the 39th overall selection, in the 2012 Major League Baseball draft. Rather than enroll at LSU, Gallo signed with the Rangers for a $2.25 million signing bonus. in 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 150 at bats over 43 games, and was named a Post-Season AZL All Star, a Topps Short-Season/Rookie All Star, and the Topps AZL Player of the Year. 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-SeasonNorthwest League, hitting.214/.343/.464 with four home runs and 26 strikeouts in 56 at bats 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 ASouth Atlantic League, leading the league with 38 home runs and 165 strikeouts in 392 at bats, and was named a Mid-Season All Star and a Post-Season All Star. He became the first teenager since Dick Simpson in 1962 to hit 40 home runs in a minor league season. Between two minor league teams, he batted.251/.338/.623 with 40 home runs and 172 strikeouts in 411 at bats. 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-AdvancedCarolina League, batting.323/.463/.735 and leading the league with 21 home runs as he struck out 64 times in 189 at bats and was named a Mid-Season All Star and a Post-Season All Star as well as the Carolina League MVP, and was promoted to the Frisco RoughRiders of the Class AATexas League in June where he was named a Post-Season All Star, a Baseball AmericaHigh Class A All Star, a Baseball America Minor League All Star, and a Topps Class A All Star. 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. Between the two minor league teams he batted.271/.394/.615 with 42 home runs and 179 strikeouts in 439 at bats.
2015
Gallo began the 2015 season with Frisco, and was named a Mid-Season Texas South All Star. 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, going 3-4 in the game. He also had a single and a double in the game. On June 5, 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. In the minors, between two minor league teams he batted.240/.342/.520 with 23 home runs and 139 strikeouts in 321 at bats. In 2015 in the majors he batted.204/.301/.417 with 6 home runs and 57 strikeouts in 108 at bats.
2016
In 2016, he spent the majority of the season in the minors, appearing in only 17 games for the Rangers. In 2016 in 25 at bats in the majors he had 19 strikeouts and one hit, a home run. With Round Rock, he batted.240/.367/.529 with 25 home runs and 150 strikeouts in 359 at bats.
2017
In 2017, Gallo won a roster spot out of spring training and played multiple positions all season for the Rangers, finishing batting.209/.333/.537 with 196 strikeouts, 11.0 at bats per home run, 41 home runs, and 80 RBIs in 449 at bats. For the season, he had the highest fly ball percentage of all major league hitters, the highest percentage of hard-hit batted balls in the majors, and the lowest contact percentage on his swings in the major leagues. He hit the third-longest home run in MLB in 2017, at 490 feet.
2018
In 2018, Gallo batted.206/.312/.498 with 12.5 at bats per home runs, and hit 40 home runs with 207 strikeouts, 74 walks, and 92 RBIs in 500 at bats over 148 games. For the second consecutive year, he had the lowest contact percentage on his swings in the major leagues.
2019
Gallo was named the AL Player of the Week for April 15-21, after hitting.478 with 4 home runs and 11 RBI over 6 games. On May 8, Gallo hit his 100th career home run versus Nick Kingham of the Pirates. He became the fastest player in American League history to reach 100 home runs, doing so in his 377th career game, a record since surpassed by Gary Sanchez, who did it in 355 games. Gallo also set a MLB record, by recording the fewest career singles at the time of reaching the 100 home run mark. On May 31, Gallo hit his first career grand slam off of Danny Duffy of the Kansas City Royals. Gallo was placed on the injured list from June 2 to June 25 with a left oblique strain. Gallo was selected as an American League reserve outfielder for the 2019 Major League Baseball All-Star Game, and hit a home run off of Will Smith in his lone at-bat. Gallo was placed on the injured list and underwent surgery to remove a broken right hamate bone on July 25, ending his season. Gallo finished the 2019 campaign hitting.253/.389/.598/.986 with 22 home runs and 49 RBI over 297 plate appearances in 70 games.
2020
On July 6, 2020, it was announced that Gallo had tested positive for COVID-19, despite being asymptomatic.
Personal life
Gallo grew up in Las Vegas and played on youth baseball teams with Bryce Harper and Kris Bryant. Mike Bryant, Kris' father, was one of Gallo's coaches growing up. Gallo's father, Tony, appeared in three games for the GCL Expos in 1974.
