Gaines made his professional debut in the NFL in 1976 with the New York Jets. In his rookie season, Gaines ran for 724 yards and hauled in 41 receptions for the Jets. Having been overlooked in the draft due to injuries, Gaines signed with the Jets as a free agent and became the first rookie ever selected as Jets' MVP. In the 1976 season, he led all NFL rookies with 724 yards rushing despite not starting until the seventh game, set a franchise record with four 100-yard rushing games in a single season, and was selected AFC Offensive Rookie of the Year by NFL Players Association. After being sidelined due to injuries in 1978, Gaines returned in 1979 as a dominant force for the Jets once again. In the 1979 season, he became the first Jet ever to lead the team in rushing and receiving twice, finishing the season fourth among AFC rushing leaders, tied for first in the AFC in yards per carry, and fumbled only once in 186 carries. The Sporting News named him the best running back in the AFC East and the most individual offensive improvement in the division in 1979. On September 21, 1980, Gaines broke a Jets franchise record with 17 catches in a single game against the San Francisco 49ers, a franchise record that still stands. Gaines is currently third, and first among running backs, in the NFL record books for receptions per game. He was the first undrafted running back to reach 500 yards rushing as a rookie, and one of six in NFL history to do so. He "wore many hats" for the Jets: although technically a running back, he was also a prolific receiver, was an intimidating blocker in short yardage situations, and played special teams. Gaines broke his leg in a game against the New England Patriots, in week 5 of the 1980 NFL season. The following year, the Jets attempted to activate him off IR, by clearing waivers. Instead, the Kansas City Chiefs claimed him. He finished his playing career in Kansas City.
NFLPA
Gaines has been employed with the NFL Players Association for 22 years, and in April 2008 was promoted to Assistant Executive Director.
Personal life
Gaines's jersey is retired at Elbert County Comprehensive High School. In 2009, he received the Elbert County Native Citizen Award by the Elbert County Chamber of Commerce.