Gilliam was a member of the Toledo Rockets for four seasons, joining the team as a walk-on and playing tight end on offense and fullback on special teams. He led the nation with four blocked kicks as a junior in 2018 and was named second-team All-Mid-American Conference.[3] Gilliam finished his collegiate career with 18 receptions for 153 yards and three touchdowns with six blocked kicks.[4]
Gilliam was signed by the Buffalo Bills as an undrafted free agent on April 25, 2020.[6] He made the Bills' 53-man roster out of training camp as a replacement for veteran fullback Patrick DiMarco, who was released by the Bills during the 2020 offseason.[7] Gilliam made his NFL debut in the Bills' season opener against the New York Jets on September 13, 2020.[8] The following week on September 20, 2020, Gilliam caught a one-yard touchdown pass from Josh Allen for his first career reception in a 31–28 win over the Miami Dolphins.[9]
On August 7, 2022, Gilliam signed a two-year, $5.2 million contract extension with the Bills.[10]
In Week 11 of the 2023 season, Gilliam was named American Football Conference Special Teams Player of the Week after forcing a fumble on the opening kickoff.[11]