The eight-player slugfest starts at 8 p.m. EDT at Coors Field and airs on ESPN.
Ohtani, who leads MLB with 33 home runs, is the top bet to win the contest. The Angels playmaker also is set to start the 2021 All-Star Game at pitcher and be the leadoff hitter for the American League on Tuesday at Coors Field.
"I'm expecting to be pretty fatigued and exhausted after these two days, but there are a lot of people who want to watch it," Ohtani told reporters Monday. "I want to make them happy so that's why I'm going to do it."
Ohtani faces Washington Nationals outfielder Juan Soto in the first round of the contest. The winner advances to face Kansas City Royals catcher Salvador Perez or New York Mets first baseman Pete Alonso -- the defending champion -- in the second round.
Gallo, the No. 2 seed in the bracket, faces Colorado Rockies shortstop Trevor Story in the first round. Gallo has the second-most home runs in the field, with 24 this season for the Rangers.
Oakland Athletics first baseman Matt Olson faces Baltimore Orioles first baseman Trey Mancini in his first round matchup.
"It isn't just me, there are seven other great hitters," Ohtani said. "I want to sit back and watch."
Alonso, Olson, Perez and Soto are among the other Top 5 favorites to win this year's slugfest in Denver. Viewers can expect to see very long home runs approaching 500 feet Monday due to the thin air at Coors Field.
"We're in Denver, so it should be fun hitting there," Gallo told reporters last week. "I think it will be a good time to have some fun and see what happens."
Ohtani's 470-foot bomb in a June 8 win over the Royals was the longest home run hit this season among the players in the field for the Home Run Derby.
He leads MLB with 17 homers of at least 420 feet.
Alonso defeated Cleveland Indians slugger Carlos Santana, Atlanta Braves outfielder Ronald Acuna Jr. and Toronto Blue Jays first baseman Vladimir Guerrero Jr. to win the 2019 Home Run Derby. The 2020 Home Run Derby was canceled due to the coronavirus pandemic.
The 2021 Home Run Derby features a single elimination format. The higher seed hits second in each round. Each batter will receive three minutes per round in the first and second rounds and two minutes in the final round.
Players receive 30 seconds of bonus time at the end of each period. They can earn another 30 second bonus if they hit at least one homer that travels 475 feet.
Ties will be broken by a 60-second swing-off, with no added time or stoppages. The winner of the Home Run Derby receives $1 million.