Gary Croteau began his trek to the NHL by playing three years of NCAA hockey with St. Lawrence University, starting in 1965 at the age of 19. Croteau was a dominant offensive force right from the start, scoring 20 goals and 31 points in just 24 games in his rookie season. He followed that up with 21 goals and 38 points in 27 games in his second season. Croteau's numbers continued to improve, and by his final year in 1967-68, he averaged better than a goal per game, scoring 21 times while collecting 40 points in only 19 games.
NHL scouts, who were not overly enthusiastic about the general level of U.S. college in those days, could not help but take notice of Croteau's exceptional offensive capabilities, and with the league having recently expanded from six to 12 teams, there was more opportunity for a wide-open style of play, and Croteau fit the bill. He was signed to a professional contract by the Los Angeles Kings in 1968. Most of that season was spent with the Springfield Kings of the AHL, but Croteau was inserted into the L.A. lineup for eleven games, where he scored five times, assisting on another.
In 1969-70, Croteau was dealt to the Detroit Red Wings, but he appeared in just ten games, picking up a pair of assists in limited ice-time duty. What does stick out in his mind is the final game of the season that year when the Wings faced the Rangers. New York and Montreal were battling for a playoff spot and goals for would come into play if the Canadiens lost their final game of the year. The Rangers won the game 9-5, but Detroit did score two empty-netters. Later that night the Montreal Canadiens, trailing 5-2 to Chicago midway through the final period, pulled goalie Rogie Vachon in an attempt to finish the season with more goals than the Rangers. The move backfired and Chicago scored five times into an empty net for a 10-2 win. Montreal missed the playoffs by two goals.
From 1970 until 1974, he got his first chance at playing full-time as a member of the lowly California Golden Seals. He was picked up by the expansion Kansas City Scouts for the 1974-75 season where he played for two seasons before the franchise moved to Denver, Colorado and were renamed the Rockies. It was in Denver that Croteau enjoyed his greatest individual success. In 1976-77, he had 24 goals and 27 assists for 51 points. Croteau remained with the Rockies until the 1979-80 season. He played one more year of pro hockey with the CHL's Fort Worth Texans before retiring at the age of 34.