Gjems-Onstad had an active career in sports. He represented the club SK Rye which he joined on 1 August 1936, and was made an honorary member of the club in 1986. He became the Norwegian junior champion in cycling in 1939. He was also an active race walker, with two sixth places in the Norwegian championships achieved between 1967 and 1969. He chaired the Norwegian Cycling Federation from 1959 to 1965 and the Norwegian Walking Association from 1967 to 1973. He was a member of the Norwegian Olympic Committee from 1959 to 1973 and a deputy board member of the Norwegian Confederation of Sports from 1965 to 1967. He was an official for Norway at the Summer Olympics in 1960 and 1972, and in 1993 he sat on the committee that organized the 1993 UCI Road World Championships. In 1967 he initiated Styrkeprøven, a yearly cyclosportive from Trondheim to Oslo. He completed the race himself more than fifteen times; the last times with the starting number "1".