In his rookie season, he alternated as the starter at left guard with veteran Fuzzy Thurston. During the 1967 season, he took Thurston's spot full-time, opposite perennial All-Pro Jerry Kramer. He started the Ice Bowl and Super Bowl II, coach Vince Lombardi's final games after nine seasons with the team.
Gillingham was the last member of the Lombardi-era Packers to be active with the franchise. By time he retired, Bart Starr, whom he blocked for when Starr was leading the Packers to victories in the first two Super Bowls, was the team's coach. Gillingham was a five-time Pro Bowler (1969, '70, '71, '73 and '74), six-time All Pro (1968, '69, '70, '71, '73, '74, and a two-time NFL First Team All Pro (1969 and '70). He was inducted into the Green Bay Packers Hall of Fame in 1982.
The only season he wasn't on offense was 1972 when head coachDan Devine inexplicably shifted him to the defensive line even though Gillingham was the team's best offensive lineman. During that campaign, the success of the Packers' offense heavily depended on a strong running attack led by MacArthur Lane and John Brockington. Devine's move, which failed when Gillingham sustained a season-ending knee injury two games into the regular season, was criticized for eventually being a factor in diminishing the team's playoff run.
Gillingham died in Little Falls, Minnesota, age 67, survived by his three sons and one daughter. Noted for his brute strength, he was one of the first players in the NFL to use weight training to stay in playing shape during the offseason. His oldest son, Karl, is a Professional Strongman and has competed in two Worlds Strongest Man competitions. Middle son, Brad, is a 6 time World Champion powerlifter with several National and World Records. Youngest son, Wade, is a former Professional Strongman and is widely regarded as having one of the best grips in the world.