He previously played in the NFL for the Los Angeles Rams, for whom he won Super Bowl LVI.
In 2018 he won the Morris Trophy, given annually to the best defensive lineman in the Pac-12 as voted on by opposing players.
Gaines was selected by the Los Angeles Rams in the fourth round (134th overall) of the 2019 NFL draft.
[5][6] Gaines won Super Bowl LVI when the Rams defeated the Cincinnati Bengals 23–20.
[7] On March 23, 2023, Gaines signed a one-year, $3.5 million contract with the Tampa Bay Buccaneers.