The gravel section is hardpacked and manageable on a road bike (although it won’t be that comfortable). The other option is to follow cycle route 2 to Skien and jump on the boat to Ulefoss. Do not cycle road 36. It’s busy with plenty of traffic. After Ulefoss, you can stick to the paved road; the traffic should be light. Once you reach Dalen, you continue following the Stavanger route over the mountains to Lysebotn. Make sure you book the Lysebotn ferry in advance. Some people have been left stranded as the ferry was full.

http://www.cyclenorway.com/routes/national-cycle-routes/overview/route-2/4887-2/
http://www.cyclenorway.com/routes/national-cycle-routes/overview/route-2/

http://www.kolumbus.no/en/travel/timetables/boats-and-ferries/lysefjorden/?fbclid=IwAR15nF8jL0ID5IIlnr6KxelnZcMbG4LmkFz6kuFSRqpSwX-bia-rDTjHmy8