He has previously played in Major League Baseball (MLB) for the Texas Rangers, New York Yankees, Los Angeles Dodgers, Minnesota Twins, and Washington Nationals.

[2] As a senior, he was the Nevada Gatorade Baseball Player of the Year and a High School All-American by MaxPreps, ESPNHS and USA Today.

[7][8] Gallo signed a National Letter of Intent to play college baseball for the LSU Tigers.

[9] 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.

[14][15] He was promoted to the Spokane Indians of the Low-A Northwest League, hitting .214/.343/.464 with four home runs and 26 strikeouts in 56 at-bats in 16 games.

[19] Over the offseason, he worked out with Troy Tulowitzki and Jason Giambi at the Philippi Sports Institute in Las Vegas.

[12] In 2017, Gallo won a major league roster spot out of spring training and played multiple positions all season for the Rangers.

[31] On May 31, Gallo hit his first career grand slam off of Danny Duffy of the Kansas City Royals.

[35] Gallo was placed on the injured list and underwent surgery to remove a broken right hamate bone on July 25,[36] ending his season.

[41] On April 9, 2021, Gallo was the only baserunner allowed by Padres starting pitcher Joe Musgrove in his no-hitter against the Rangers, getting hit by a pitch in the fourth inning.

[44] On July 29, 2021, the Rangers traded Gallo and Joely Rodríguez to the New York Yankees in exchange for Josh Smith, Glenn Otto, Trevor Hauver, and Ezequiel Durán.

[48] Gallo's performance dropped off dramatically in 2022, continuing what was described in the New York Post as a "nightmare tenure in The Bronx."

On November 3, the Nationals declined the mutual option on Gallo's contract, making him a free agent.

[55] Gallo grew up in Las Vegas and played on youth baseball teams with Bryce Harper and Kris Bryant.

Gallo's father, Antonio (Tony), is the son of Italian immigrants from Sciacca, Sicily, and was raised in Bensonhurst, Brooklyn.

During the delay of the 2020 Major League Baseball season due to the COVID-19 pandemic, Gallo stayed in his Dallas apartment and set up a batting cage in his living room.