Gillespie began his coaching career in 1995 as an assistant coach at his hometown Stephenville High School under head coach Art Briles. In 2008, he became the head coach of Stephenville where he led the team to a state championship in 2012.[1][2]
In 2016 he was hired as the linebackers coach at Tulsa, and was promoted to defensive coordinator in 2019.[2][3]
In 2022, he was hired by first year head coach Sonny Dykes to be the defensive coordinator of TCU.[4][5] In his first season at TCU, Gillespie was credited for turning around the Horned Frog's defense and helping lead the team to an 13–2 season.[6] For his success in 2022, Gillespie was named a finalist for the Broyles Award.[6] However, the TCU defense struggled in 2023, missing a bowl, leading to Gillespie's dismissal on December 10, 2023.[7][8]
In 2024, he was hired to be the new head coach at Midway High School.[9]