We have just done this, and it is spectacular. From Sangüesa, you follow the road to Liedana, then you follow the old railway Camino verde, which is actually way marked through the gorge. You will probably stay in the hotel Iru-Bide unless you plan to walk through in one day, not advisable. From the roundabout next to the hotel there are yellow arrows up the road, off to the right on a track to Aldunate where the arrows stop. Following a Wikiloc track, we turned right onto the N240 (a very little used road) to join the main Camino route just outside Izco. Undermanager posted recently and seemed to do the same thing and that is the Gronze route.
PS we suspect there has been a route change after Aldunate, probably a landowner blocking the route, but if you stay on the N240, you’ll be fine.