[2] His younger brother Jake Gelof played baseball as a third baseman for the University of Virginia, was drafted in the second round of the 2023 draft by the Los Angeles Dodgers, and now plays in the Dodgers organization; Jake was Zack's teammate in both high school and college.

[3] Gelof grew up in Rehoboth Beach, Delaware, where he attended Hebrew school in the Seaside Jewish Community.

[3] He also played soccer for the high school, and finished his career as the leading goal scorer in the State of Delaware for his four-year career span 2015-18 (61 goals; 16 assists), and was four-time All-Conference, three-time All-State, and as a senior was named Conference Player of the Year.

[5] He was selected in the 38th round of the 2018 Major League Baseball draft by the Cleveland Indians, but Gelof opted not to sign with the team.

[10][15] As a sophomore in 2020, Gelof batted .349/.469/.746 (leading the ACC) with 24 runs scored (leading the ACC; 4th in the NCAA), six doubles (7th), two triples (4th), five home runs (6th), 18 RBIs (8th), and 13 walks (10th) in 18 games at third base before the season was cut short due to the coronavirus pandemic.

[3] He played for the High Point-Thomasville HiToms of the collegiate summer Coastal Plain League after the season, batting .364/.442/.636.

[18] Virginia played in the 2021 College World Series, and he was named the third baseman on the All-Tournament Team after hitting .583.

[23] In 2021 Gelof was assigned to the Rookie-level Arizona Complex League Athletics to start his professional career, where he played in one game.

[25] He finished the season with the Las Vegas Aviators of the Triple-A West, where while nearly six years younger than the average player in the league and only two months after playing college baseball, he batted 7-for-12 with 6 RBIs.

[29][24] He missed almost seven weeks with a torn labrum and subluxed left (non-throwing) shoulder suffered in May when he dove for a ground ball, but playing for the Midland RockHounds of the Double–A Texas League he hit .271/.356/.438 with 13 home runs and nine stolen bases in 11 attempts in 354 at bats over 83 games.

[36] The next day, Gelof was formally selected to the 40-man roster and promoted to the major leagues for the first time, and he made his MLB debut for the A's on July 14.

Gelof hit his first major league home run on July 22, 2023, against Ryne Stanek of the Houston Astros.

[48] Gelof earned $388,000 from the MLB pre-arbitration bonus pool allocated for players with between 0-3 years of service time, who played on league-minimum contracts, and who were paid significantly less than their 2023 on-field production would warrant.

[50][51] In 2024 with the A's, Gelof batted .211/.270/.362 in 497 at bats, with 60 runs, 20 doubles, 17 home runs, 49 RBIs, 3 sacrifice hits (9th in the AL), and 25 stolen bases (8th) in 28 attempts (his 89.29% stolen base percentage was 9th in the league), with a 20.2 power-speed number (9th), and led the AL with 188 strikeouts and in strikeout percentage (34.4%).

[55] That month they hosted an inaugural youth sports camp for children ages 8-13 at Cape Henlopen High School in Lewes, Delaware.