Gabriel passed for 2,951 yards in three years as a starting quarterback at North Carolina State. A first-round draft choice of the NFL's Los Angeles Rams in 1962, he became a starter for a time during his rookie season, but was primarily a backup to Bill Munson in 1964 and 1965.

When George Allen took over as head coach in 1966, he made the 6-foot-4, 220-pound Gabriel the starter once more and Gabriel responded by completing 217 of 397 passes for 2,540 yards and 10 touchdowns.

Gabriel was the consensus NFL player of the year in 1969, when he completed 217 of 399 attempts for 2,549 yards and 24 touchdowns while throwing only 7 interceptions. The Rams won the Coastal Division title that year but lost 23-20 to the Minnesota Vikings in the first round of the playoffs.

Hampered by injuries during the next three seasons, Gabriel was traded to the Philadelphia Eagles in 1973. He had one fine year with the Eagles, but was once more troubled by recurring injuries in 1974 and 1975. He underwent knee surgery in 1976 and retired after the 1977 season.

In his 16 years as a professional, Gabriel completed 2,366 of 4,498 passes for 29,444 yards and 201 touchdowns. He also rushed for 30 touchdowns.