[1] Being a student in Oslo at the outbreak of World War II, Gjelsvik participated in the resistance movement from 1940.
degree in 1953 on the thesis Metamorphosed Dolerites in the Gneiss Area of Sunnmøre on the West Coast of Southern Norway.
He participated in Polar research, as a member of several committees, such as Det interdepartementale Polarutvalg and Polarrådet.
He took part in the developing Norway's Resistance Museum, chairing its advisory board from 1964 to 1973, and was a member of the Fram committee from 1960 to 1997.
[1] Gjelsvik was decorated Knight, First Class of the Royal Norwegian Order of St. Olav in 1975, and Commander in 1984.