Gerela graduated from Kalani High School in Honolulu, and played college football at New Mexico State University. He was selected in the fourth round of the 1969 AFL Draft by the Houston Oilers and played for them in the American Football League in 1969, as well as their first NFL season of 1970. He signed with the Steelers before the 1971 season, and quickly became popular with Steelers fans, inspiring a fan club, "Gerela's Gorillas." He led the AFC in scoring in 1973 and 1974, and was named to the Pro Bowl in 1973 and 1975.
Gerela was a member of the Steelers' Super Bowl championship teams in 1973-74, 1974–75, and 1978–79, but was released by the Steelers before the 1979-80 season in favor of Matt Bahr. He appeared in three games for the San Diego Chargers in 1979 before a torn groin muscle ended his career.