[1] He scored 42 points in a regular-season win over Neumann Goretti featuring Kentucky recruit Quade Green.

He signed a letter of intent with Villanova on April 14, 2017, joining Dhamir Cosby-Roundtree and Jermaine Samuels in the class of 2021.

[3] He provided quality minutes and had eight points including two three-pointers in a February 1 win over Creighton due to the absence of Phil Booth with a similar hand injury.

[8] Coming into his junior season, Gillespie broke his nose in practice, forcing him to wear a mask for several games.

[14] Coming into his senior season, Gillespie was named to the Bob Cousy Award watchlist as well as Preseason First Team All-Big East.

[17] At the conclusion of the regular season, Gillespie was named Big East Co-Player of the Year alongside teammate Jeremiah Robinson-Earl and Seton Hall's Sandro Mamukelashvili.

In his Summer League debut, Gillespie scored 11 points, six rebounds, and four assists in a 78–85 loss to the Minnesota Timberwolves.

[24] On July 30, 2022, the Nuggets announced that Gillespie had undergone surgery for a lower leg fracture and that he would be out indefinitely.

In the summer of 2019, Gillespie was a part of the United States National team who competed at the Pan American Games in Peru.