The Guisane valley, named after the river that drains its waters, is the valley of the Hautes-Alpes that descends from the Lautaret pass and leads eastward to Briançon in the Durance valley. It is therefore part of the Briançonnais.
Photo (c) Altimood - altimood.com/grand-tour-ecrins
See also on GR54 :