A star player in high school, he signed a pro football contract with the Cleveland Browns just after graduation but left training camp at Hiram College, in Hiram, Ohio and went to Canada to play where he was team MVP for the Sarnia Imperials and the Kitchener-Waterloo Dutchmen of the Ontario Rugby Football Union. He joined the Canadian Football League’s Hamilton Tiger-Cats who he helped lead to a 1957 Grey Cup victory, and played for the Saskatchewan Roughriders and Toronto Argonauts.
Traded to the Buffalo Bills of the fledgling American Football League, Gilchrist played fullback and kicked, though he insisted he could have played both ways. He was the first American Football League player to rush for 1,000 yards in a season: 1,096 in a 14-game schedule in 1962; earning him league MVP honors. Gilchrist rushed for a professional football record 243 yards and five touchdowns in a single game against the New York Jets in 1963. Though he was only with the Bills for three years (1962-1964), he remains the team's fifth leading rusher all-time, and led the league in scoring in each of his three years as a Bill. Gilchrist ran for 122 yards in the Bills' 1964 American Football League championship defeat of the San Diego Chargers, 20-7. His 4.5 yds/rush average is second as a Bill only to O.J. Simpson.
In an early civil rights victory for black athletes, Gilchrist led a successful boycott of New Orleans as the site of the 1965 American Football League All-Star game. He is the only athlete to turn down being enshrined into the Canadian Football Hall of Fame and Museum, because of what he described as racism and exploitation by management. Gilchrist frequently was at odds with team management, the American ones working for Canadian teams, he told a reporter from the London Free Press, saying that most of the problems he encountered were a result of his standing up for principles at a time when black athletes were expected to remain silent.
Gilchrist also played for the Denver Broncos in 1965 and 1967, and the Miami Dolphins in 1966. Gilchrist was selected to the second team, All-Time All-AFL, and he is member of the American Football League Hall of Fame.