Gabriel was born to Edna Mae Wyatt and Roman Ildonzo Gabriel Sr., a Filipino immigrant, in Wilmington, North Carolina. Gabriel grew up poor and suffered from asthma, but he played high school football at New Hanover High School and graduated in 1958. He went on to star at quarterback at North Carolina State University in Raleigh.

A two-time All-American and two-time ACC Player of the Year (1960–61), Gabriel finished his college career holding virtually every Wolfpack passing record. An academic All-American, Gabriel saw his jersey retired after his senior season and then presented to him by North Carolina governor Terry Sanford on January 20, 1962, at halftime of an NC State-Maryland basketball game in Reynolds Coliseum. As captain of his team, Gabriel set 22 school and nine conference football records. He threw for 2,961 yards and 19 touchdowns.

From 1962 through 1965, Gabriel had a difficult time securing a starting quarterback job. Ram coaches gave Zeke Bratkowski or Bill Munson the nod over Gabriel. However, due to other quarterbacks slumping or being injured, Gabriel did get to start 23 games from 1962 through 1965. The Rams record in those games was 11 wins, 11 losses, and one tie. Although his record as a starter was average, the other Rams quarterbacks who started the other 33 games combined record was only four wins, 27 losses and two ties. Gabriel’s significant wins include a 1965 victory to beat the eventual Champion Green Bay Packers and the 11–3 Cleveland Browns.

When George Allen took over for Harland Svare to coach the Rams in 1966, one of his first moves was to make Gabriel the #1 starter. Gabriel started all 14 games and the Rams went 8-6, their first winning season since 1958. In 1967 the Rams went 11–1–2 and made the playoffs as NFL Coastal Division champions. Gabriel was named the AP Offensive Player of the Week the last two weeks of the season. In week 13, needing a win to keep their playoff hopes alive, Gabriel was 20 for 36 with 3 touchdowns (including the game winner in the last minute) in a 27–24 come from behind win over the defending champion Green Bay Packers. The next week, in a game against the Baltimore Colts that would decide the division title, Gabriel completed 18 of 22 passes with 3 touchdowns as the Rams won 34-10. The 1967 Rams finished as the highest scoring team in the NFL, but were eliminated from the playoffs by the Packers 28-7. Gabriel threw for 2,779 yards and 25 touchdowns and was a Second-team All-Pro and a Pro Bowler.

Gabriel wore the number 18 with the Rams and the number 5 with the Eagles. In the professional ranks Gabriel went on to play 16 seasons in the NFL, splitting time with the Los Angeles Rams (1962–72) and the Philadelphia Eagles (1973–77). He was awarded the NFL Most Valuable Player Award in 1969 and earned Pro Bowl spots in 1967, 1968, 1969, and 1973. When he retired, he ranked as the Rams’ all-time passing leader with 22,223 yards and 154 touchdowns (1,705 com./3,313 att.) and threw for 7,221 yards and 45 touchdowns (661 com./1,185 att.) with the Eagles. In 1973, he led the NFL with 3,219 yards and 23 touchdown passes, for which he was awarded the NFL Comeback Player of the Year Award. As of the end of the 2016 NFL season, he still holds the Rams’ career records for touchdown passes (154), passes attempted (3,313), and wins by a starting quarterback (74).

In 1969 the Rams opened the season with an 11-game winning streak (still a team record), before suffering their first loss to the 10–1 Minnesota Vikings in Los Angeles by a score of 20–13. With the division clinched and the undefeated record gone, coach Allen decided to rest many of his starters and the Rams lost their last two games to finish 11–3. In a rematch with the Vikings in the playoffs in Minnesota, the Rams lost, 23–20. For the season, Gabriel threw 24 touchdowns and only seven interceptions and was named the NFL’s Most Valuable Player by the AP and NEA, the Player of the Year by the UPI and was voted All-Pro and to the Pro Bowl.

Gabriel had a brief career in movies, playing a prison guard in Otto Preminger’s 1968 spoof Skidoo and a Native American named “Blue Boy” in the 1969 John Wayne and Rock Hudson film, The Undefeated. He had previously appeared as a headhunter in the November 14, 1966 “Topsy-Turvey” episode of CBS’ Gilligan’s Island. With several of his Rams teammates, he made a cameo appearance as a football player in the 1965 Perry Mason episode, “The Case of the 12th Wildcat”, as well as in 1970 on an Ironside episode, “Blackout.” He was also on a 1978 episode of Wonder Woman, “The Deadly Sting.”

In 1970, the league realigned, putting the Rams in contention with the San Francisco 49ers for the new NFC West Division title. After an upset loss at home to the lowly New York Jets (who were without the injured Joe Namath) in which Gabriel threw three interceptions, the Rams won three straight, including a crucial 30–13 win over the 49ers to take over first place. But a 28–23 week 13 loss to the Detroit Lions in the first Monday night football game in L.A. (despite over 300 yards passing from Gabriel), put the Rams back in 2nd place and left them a half game behind the Lions for the wild card playoff spot. The Rams won their finale at the New York Giants 31-3 (eliminating the Giants from playoff contention) but failed to make the playoffs as the Lions won to secure the wild card spot, and the 49ers won to clinch the NFC Western Division title. From 1967-70, Gabriel led the Rams to a 41-14-4 overall record, and was named to three Pro Bowls during that four-year span.

In 1971, the veteran Rams began to show their age and Gabriel missed parts of every game due to knee and shoulder injuries. In addition, coach George Allen left for the Redskins after a long running dispute with general manager Dan Reeves. Still, the Rams, despite playing the league’s toughest schedule, faced almost the same situation as in 1970. After falling behind the 49ers in week 7, they rallied back to take the division lead going into another week 13 Monday night game in L.A. This time, the opponent was George Allen’s Redskins. After falling behind 31–10, Gabriel led the Rams back to within 31-24 and was driving to a possible tying score when he was intercepted; it was returned for a touchdown and the Redskins won. Once again, the Rams fell into 2nd place behind the 49ers and behind the Redskins for the wild card berth. Despite winning in Pittsburgh in week 14, the 49ers came from behind to beat the Lions, 31–27 and win the division.

In 1972, Gabriel’s knee and shoulder injury problems got worse. After making 89 consecutive starts over eight seasons, he missed two games and lost playing time in all 12 others. Still, after a Monday night win in San Francisco in week 12, the Rams regained 1st place. But losses to the Cardinals and Lions in the final two weeks, in addition to a week 11 loss to the woeful Saints, doomed their season. The Rams finished 6–7–1 and coach Tommy Prothro was fired.

After the 1972 season, the Rams hired Chuck Knox as their new coach, and obtained John Hadl to be quarterback. After he threatened to accept a $100,000 contract with the Las Vegas Casinos of the Southwestern Football League in April 1973, Gabriel was traded from the Rams to the Philadelphia Eagles for Harold Jackson, Tony Baker, a 1974 first-round selection (11th overall–John Cappelletti) and first- and third-round picks in 1975 (11th and 67th overall–Dennis Harrah and Dan Nugent) on June 8, 1973.

Gabriel was the last football coach at Cal Poly Pomona, where from 1980 to 1982 his teams compiled an 8–24 record. On November 26, 1982, he resigned to become offensive coordinator with the Boston Breakers of the USFL.

Known for his arm strength, he also played baseball and was voted the best amateur athlete in the Carolinas. In a three-year career, he passed for 20 touchdowns and ran for 15. The Atlantic Coast Conference’s 50th Anniversary Football Team was announced in 2003 and Gabriel was among the top 50 players in the history of the ACC to be listed. Gabriel was inducted into the College Football Hall of Fame in 1989.