Gudbrandsdalen (gŏŏd´brändsdäl´ĕn), valley region of Oppland co., S central Norway. It extends c.100 mi (160 km) from the Dovrefjell southeastward to Lillehammer on Mjøsa lake. A rich agricultural and timber district, the valley is also a tourist area. The Lågan River, which traverses the valley, is a valuable source of hydroelectric power. The valley has long been an important trade route and a main invasion route through S Norway and has a rich history; many Gudbrandsdalen farmers trace their ancestry back to saga times. Much of the action in Ibsen's Peer Gynt is set there.