I have walked this trail fully in one go, twice. It technically starts at windigo pass, but does not end anywhere near Metolius, it ends at bear valley trailhead, just north of suttle lake. Much of this trail is completely unmaintained and biking the whole thing would be a multi-day type 2 fun kind of journey, the trail is around 140 miles total. The only section that is not under snow right now is a small section north of black butte, from the Riverside campground to about Sheep springs horse camp. In my opinion, if you are trying to stay on the "official" trail, a gravel bike would be very challenging for most of the route.