By the
1963–64 season, Hadfield had secured a place in the Rangers' lineup as an enforcer. Over time he concentrated more on scoring than on fighting, especially with the feared enforcer
Reggie Fleming on the team. Hadfield had the first of five 100 penalty minutes in the 1963-64 season. He joined teammates
Jean Ratelle and
Rod Gilbert to form the famous
GAG line (which stood for "goal a game"). From the
1967–68 season on Hadfield always scored at least 20 goals in any full season. Hadfield's best season was
1971–72. Named the team's captain after the trade of longtime captain
Bob Nevin, he became the first Ranger - and only the sixth NHL player - to score 50 goals in a season, nearly doubling his previous best marks; with his linemates Ratelle and Gilbert, the GAG Line collected a total of 139 goals and 325 points (which likely would've been more as Ratelle had to miss 15 games due to injury) on their way to the
Stanley Cup Final, the first appearance for the team since 1950. In his only Stanley Cup Final appearance, Hadfield had just one goal and two assists as the Rangers lost to the
Boston Bruins in six games. Hadfield reflected on the effectiveness of the line in a 2018 interview: It’s a combination of things. I think as a line we each had our own particular role that we had to play. We were able to put it together. It’s not like today. Back then we were together for I think six, seven, eight years. Today you don’t even see two guys playing together for any length of time – plus practices. Jean could stick-handle and control the puck, Rod could shoot it. My role was to be aggressive and get into the corners and dig the puck out. And really that’s what it was, really repetitious, It was there every day, so we knew exactly where everybody was. You look back today, and I’m not a guy that’s looking for anything special (in terms of accolades) but when they put the line together we all had an idea of what we were going to be doing. The line holds the record in the National Hockey League for the most points on one line. That’ll never be broken because, like I said, two guys don’t even stay together, never mind three. But I don’t dwell on stuff like that because the main goal is to win the Stanley Cup. Personal shit didn’t really matter. It was nice, yeah, but our goal was to go into training camp and prepare ourselves to win that Cup, and anything less was, it was a void there. The Rangers signed Hadfield to a controversial and lucrative five-year contract for $1 million in the summer of 1972 to deter him from defecting to the newly created
World Hockey Association (which offered him a $1 million contract for four years). Hadfield was included on the roster for the
Summit Series in September 1972 for Canada (as coached by
Harry Sinden) against the Soviet Union but played in just two games and recorded no goals or assists. Not long after
Team Canada arrived in
Moscow for the final four games of the series, Hadfield, alongside
Rick Martin and
Jocelyn Guevremont, left the team and went home for what they felt was a lack of playing time. The 1972-73 season saw Hadfield play in 63 games and record just 28 goals with 34 assists. In what ended up being his final season for New York in 1973, he scored 27 goals with 28 assists while having just one playoff goal in the
Stanley Cup playoffs. He was traded to the
Pittsburgh Penguins for
Nick Beverley on May 28, 1974. He scored thirty goals in each of his two full seasons for the Penguins although he was plagued by injuries and weight troubles. Near the end of the
1975–76 season he sustained a knee injury which forced his retirement. ==Legacy==