During his career as a rower, Gunnar Sandborg won five Norwegian national championships, while his greatest international moment came when he was selected to represent his country in the coxed fours at the 1948 London Olympics. There, alongside Arne Serck-Hanssen, Sigurd Grønli, Willy Evensen, and Thoralf Sandaker, he lost his round one heat against the upcoming bronze medalists from Denmark, then won his repêchage, and was subsequently eliminated in the quarter-finals. By career he worked in the paper industry until his retirement in 1995, but also served abroad for nearly two decades as a trade consul in Kenya, Tanganyika, and Uganda. Outside of rowing, he also competed in cross-country skiing and sailing.