Gainer attended Plant City High School, where he received All-conference recognition as a senior running back. He accepted a football scholarship from Florida A&M University. As a sophomore, he was lost for the season with a knee injury. In his junior year, he posted 235 rushing yards. As a senior, he led the team in rushing yards and the Mideastern Athletic Conference in rushing touchdowns.
Professional career
Los Angeles Raiders
Gainer was selected by the Los Angeles Raiders in the eighth round 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. On September 9, he was signed to the practice squad. He was released on November 29.
Cleveland Browns
On December 13, 1989, he was signed by the Cleveland Browns to their practice squad. The next season, he made the regular season roster and played in all 16 games as a backup fullback, while leading the team with 18 special teams tackles. His first start came against the New York Jets in place of an injured Kevin Mack, but he suffered a knee injury in the second half of the game. He was released on August 26, 1991.
On October 15, 1992, he was signed as a free agent by the Dallas Cowboys to help on the special teams units. On November 23, he was placed on the injured reserve list with a strained right hamstring and was re-activated on December 28. 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 rushed for 29 yards and scored the last touchdown. He had 2 carries for one yard in relief of Smith during the Super Bowl. 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. He was the fourth-string running back and appeared in 11 games. On May 10, 1994, he was traded to the Los Angeles Raiders in exchange for past considerations.
Los Angeles Raiders (third stint)
Gainer was released by the Los Angeles Raiders on August 28, 1994.