Coming into his senior season, Gillaspia moved from linebacker to fullback as Jimbo Fisher shifted to a pro-style offense.

In his final play, Gillaspia scored a 13-yard rushing touchdown in A&M's 52–13 win over NC State in the Gator Bowl, the first touchdown by a 12th man in program history[2] Gillaspia was drafted by the Houston Texans in the seventh round, 220th overall, of the 2019 NFL draft.

In the American Football Conference wild card game, Gillaspia made a crucial block on two Bills defenders to set up Deshaun Watson for a touchdown run in a 22–19 overtime victory.

[5] In Week 3 of the 2020 regular season, Cullen Gillaspia caught his first career reception, against the Pittsburgh Steelers.

[12] He was placed on injured reserve on November 12 after suffering a knee injury that would require surgery.