Hi John, unfortunately, there are no gravel routes from Trondheim to bodø. Whatever you have seen on a map is hiking paths through exceptionally difficult terrain. What many Norwegian bikepackers do is head into sweden and cycle the gravel roads up and then cross back into Norway further north. Here is an example of a famous gravel route through Sweden: https://www.komoot.com/collection/1070173/7-600-km-across-the-continent-european-divide-trail