[1] Gran finished his secondary education at Kristiania Cathedral School in 1888, and graduated from the Royal Frederick University with the cand.real.

He originally studied phycology under Nordal Wille, but from 1897 he worked with marine zoology under Johan Hjort, and together with hydrographer Fridtjof Nansen.

[1] Gran especially concentrated on planktology, and took the doctorate in 1902 with the thesis Das Plankton des norwegischen Nordmeeres, following field study in the Norwegian Sea.

He worked as a research fellow at Bergens Museum from 1901 to 1905 and as a professor of botany and director of the University Botanical Garden in Kristiania from 1905 to 1940. Notable publications include the German-language Diatomeen (1908), Pelagic Plant Life (1912) and A quantitative study of the phytoplankton in the Bay of Fundy and Gulf of Maine (with Trygve Braarud, 1935).

[1] Gran was also a choir singer, a keen horticulturalist (chairman of the Norwegian Horticulture Society from 1908 to 1938), vice chairman of the Norwegian-British Friendship Society from 1921 to 1929 and a member of the Oxford Group.