Memphis Hustle (2021) After going undrafted in the
2020 NBA draft, Gillespie signed a training camp contract with the
Dallas Mavericks, but didn't make the final roster. On January 11, 2021, the
Memphis Hustle selected Gillespie with the 2nd pick overall in the 2020–2021
NBA G League draft. In 15 games, he averaged 10.5 points, 10.3 rebounds and 2.3 blocks in 27.8 minutes while shooting .570 from the field and recording six double-doubles.
Toronto Raptors (2021) On April 8, 2021, Gillespie signed a 10-day contract with the
Toronto Raptors. On April 14, in a 122–117 win over the
San Antonio Spurs, he scored 9 points, grabbed 8 rebounds and recorded 2 blocks, all career-highs. Two days later, he reached a new career-high of 10 points, with seven rebounds, in a 113–102 win against the
Orlando Magic. On April 18, Gillespie signed a second 10-day contract and tied his career-high with 10 points, two rebounds, one assist and one steal, going a perfect 4 of 4 from the field in a 112–106 win against the
Oklahoma City Thunder. Three days later, he had four points, five rebounds, and a career-high five blocks in a 114–103 win against the
Brooklyn Nets. On April 28, Gillespie signed for the remainder of the season. He was represented by sports agency Beyond Athlete Management. On May 2, 2021, Gillespie had a new career-high of 11 points, with seven rebounds and one steal, in a 121–114 win over the
Los Angeles Lakers. On October 13, 2021, Gillespie was waived by the Raptors.
Return to the Hustle (2021) On October 23, 2021, Gillespie re-signed with the
Memphis Hustle. In 12 games, he averaged 11.4 points, 12.2 rebounds, 1.0 assists, 2.75 blocks and 1.17 steals in 33.2 minutes per game, leading the team in rebounding and leading the entire league in blocked shots.
Orlando Magic (2021–2022) On December 21, 2021, Gillespie signed a 10-day contract with the
Orlando Magic.
Third stint with the Memphis Hustle (2022) On January 10, 2022, Gillespie was reacquired and activated by the
Memphis Hustle.
Bayern Munich (2022–2023) On July 25, 2022, Gillespie signed with
Bayern Munich of the German
Basketball Bundesliga until 2024.
Crvena zvezda (2023–2024) On December 18, 2023, Gillespie left Bayern Munich and signed with Serbian club
Crvena zvezda for the rest of the season. On June 10, 2024, he parted ways with the team.
New Zealand Breakers (2024) On July 25, 2024, Gillespie signed with the
New Zealand Breakers of the Australian
National Basketball League (NBL) for the
2024–25 season. On September 15, he received a one-match suspension after an incident in which he headbutted
Shaun Bruce of the
Sydney Kings. He was granted a release from his contract on November 18, 2024.
Olimpia Milano (2024) On November 19, 2024, Gillespie signed with Italian team
Olimpia Milano for the remainder of the 2024-25 season. On March 6, 2025, with just more than two minutes of playing time in the second half, Gillespie suffered a scary blow to the head landing on the court after colliding with
Devon Hall on a loose ball. He was unconsciousness for a few seconds and then left the court on a stretcher.
Galatasaray (2025–present) On September 22, 2025, Gillespie signed with
Galatasaray. ==Career statistics==