There are no long-distance gravel routes along the fjords. If you’re heading to Trondheim via Kristiansand, then you need to head east towards Oslo and pick the NGRs: https://cyclenorway.com/en/national-gravel-routes/

If you want spectacular fjord and mountain roads, the Fjord Norway route is the one to do, but you need to be aware of tunnels, busy roads, and challenging mountain passes in bad weather: https://cyclenorway.com/en/routes/fjord-norway/