A five time Pro Bowler, he was considered one of the quickest and most-feared pass-rushers of his generation, he had 100.5 quarterback sacks in only his first hundred starts. Gastineau also held the record of most sacks in a single season, 22 for sixteen years before Michael Strahan broke it in 2000.

Gastineau retired in the middle of the 1988 season to spend more time with actress Brigitte Nielsen but he later admitted the real reason was his fear of failing another drug test. In 1990, Gastineau tried a comeback with the British Columbia Lions in the Canadian Football League but played in only four games. A year later, Gastineau pursued a career in boxing where he finished his career with fifteen wins and two losses. Gastineau has had several run-ins with the law, including domestic violence and spending eleven months in prison for parole violatons in 1999.

Gastineau was born in Ardmore, Oklahoma. When he was seven his family moved to the town of Springerville, Arizona where he played high school football for the Round Valley Elks.

He played defensive end for Arizona State University then East Central University in Ada, Oklahoma where he had 27 quarterback sacks in his college career. He became that school's first-ever draft pick, selected in the second round by the New York Jets in the 1979 NFL Draft.

In New York, Gastineau became the leader of the New York Sack Exchange which included former teamate Joe Klecko. Gastineau made the Pro Bowl five straight seasons (1981-85) and finished his 10-year career with 107½ sacks, the Jets all-time record. His best season was 1984, when his then-record setting 22 sacks earned him NFL Defensive Player of the Year honors. He also recorded 69 tackles and one fumble recovery. He was selected the MVP of the Pro Bowl game that season.

Gastineau was nationally famous for doing his signature "Sack Dance" after sacking an opposing quarterback. But he had to stop when the NFL declared it "unsportsmanlike taunting" in March 1984 and issued a fine for whoever does it.

Gastineau seemed poised to return to his old self, leading the AFC in sacks seven weeks into the 1988 season. However, he shocked the football world by announcing his retirement to spend more time with actress Brigitte Nielsen, to whom he had previously announced his engagement, even though he was still married to his first wife, Lisa Gastineau; he and Nielsen later had a son.

Gastineau tried a comeback, in the Canadian Football League in 1990. He signed a two-year contract with the British Columbia Lions, but got injured and was released after only four games.

In 1991, Gastineau pursued a career in boxing. His first fight was against Derrick Dukes. Gastineau won with a first round knockout. Dukes, a professional wrestler, later admitted he took a dive. TV newsmagazine show 60 Minutes interviewed several others that fought Gastineau and were told to take dives so that it made Gastineau look good. His career ended in 1996 when he lost to another former football player, Alonzo Highsmith. His record in boxing was fifteen wins, two losses, and one no contest.

He admitted years later the real reason why he retired was fear of failing another drug test and the Jets learning he was using anabolic steroids. Convicted for drug possession in 1993, Gastineau has had several run-ins with the law, including domestic violence. In 1999, Gastineau spend eleven months behind bars in Rikers Island for parole violations. His ex-wife Lisa and their daughter Brittny Gastineau (who he has not seen in 17 years) are the stars of the E! reality television show, The Gastineau Girls.