Gainer was selected by the Los Angeles Raiders in the eighth round (205th overall) of the 1989 NFL draft.
Although officially it is recorded that the Raiders waived him before the start of the season, it was actually his decision to leave the team.
[8] On October 15, 1992, he was signed as a free agent by the Dallas Cowboys to help on the special teams units, where he had 2 tackles.
After Curvin Richards was released for fumbling in the last regular season game against the Chicago Bears, Gainer was also used to backup Emmitt Smith during the playoff run to Super Bowl XXVII.
In the playoff 34–10 win against the Philadelphia Eagles, he entered the fourth quarter, rushing for 29 yards and scored the last touchdown.
In 1993, he got the opportunity to compete with rookie Derrick Lassic for the starting running back job, while Smith was in a contract holdout.