Gagne started wrestling in 1972 in his father Verne Gagne's promotion, the American Wrestling Association (AWA). Gagne formed the team The High Flyers with Jim Brunzell, and on July 7, 1977, they defeated Bobby Duncum and Blackjack Lanza to win the AWA World Tag Team Championship. The Flyers successfully defended the title in rematches with the former champions as well as other contenders within the AWA ranks for nearly 15 months before they had to vacate the title due to an injury that Jim Brunzell sustained. The title was awarded to Pat Patterson and Ray Stevens in late September 1978. The Flyers regained the titles in June 1981 from The East-West Connection (Adrian Adonis and Jesse "The Body" Ventura). The Flyers held the title for a little more than two years before dropping it to Ken Patera and Jerry Blackwell on June 26, 1983. Though the Flyers came close to regaining the title on several occasions, they never recaptured it. The team parted ways shortly thereafter, with Gagne entering into singles competition while Brunzell worked frequently as both a singles and tag team wrestler before departing the company in 1985.
From 1984 to 1986, Gagne was involved in a feud with Sheik Adnan Al-Kaissie's army, which at the time included King Kong Brody. The feud also saw Greg's retired father Verne return briefly for a number of matches. Meanwhile, in 1985, Gagne began an ill-fated and short-lived metamorphosis, becoming a member of Sgt. Slaughter's Cobra Corps. The idea saw a fatigues-clad Gagne teaming with Slaughter.
In the late 1980's, Gagne feuded with Curt Hennig, Ron Garvin, and Larry Zbyszko and briefly resurrected his feud with Sheik Adnan Al-Kaissie. His feud with Hennig over the AWA Title only heated up when both fathers, Verne and Larry Hennig, became embroiled in the feud interfering on behalf of their sons during many of their matches. Gagne apparently won the title on several occasions only to have the belt returned to Hennig on technicality.
After SuperClash III, the AWA stripped Jerry Lawler of the AWA World Title. It was then decided that the new champion would be determined in a battle royal in February 1989. Many fans expected either Gagne or Slaughter to win the title that night and the crowd reacted with shock when both were eliminated late in the match. The crowd was even more stunned when Larry Zbyszko became the new champion by eliminating Tom Zenk to end the match. Gagne challenged Larry Zbyszko for the AWA World Heavyweight Championship at War in the Windy City, but was unsuccessful. It was also around this time the Sheik Adnan Al-Kaissie attacked Verne Gagne during an interview. As a result, Gagne turned his attention to Adnan in a feud that eventually involved Kokina Maximus. Gagne's wrestling career ended during a match for the vacated AWA World Tag Team Title. Gagne and Paul Diamond were facing The Destruction Crew in the finals when Adnan and Kokina came to the ring and confronted Gagne. Kokina slammed Gagne on the cement floor outside the ring and then splashed his leg, apparently doing damage to Gagne's knee. Gagne retired due to the injury and began to work in the broadcast booth and in production. Publicly, he enlisted the help of Jerry Blackwell to exact revenge on Adnan and Kokina. Despite the build up, the match never occurred.
Though despite working the majority of his career for his father's promotion, Gagne made some rare appearances in the World Wrestling Federation. Gagne's first appearance in the WWF was in 1977, in a match at Madison Square Garden, where he defeated Johnny Rodz. Gagne's second appearance in the WWF was in April 24, 1980, where he defeated preliminary wrestler Jose Estrada in a match at MSG. Gagne's third and final appearance in the WWF was at the Showdown at Shea event in August 9, 1980, where he defeated Rick McGraw.
Greg inducted his father, Verne Gagne, into the WWE Hall of Fame on April 1, 2006.