The nearest airport is Stockholm (ARN). From here grab any train that goes to Borlänge Central (2 hours 5 minutes), then transfer to another train and go one stop to Gagnef (15 minutes). The festival is then a 23 minute walk away.
Gagnef station is accessible by train from various European cities but will most likely require at least one transfer.
As the festival campgrounds are generously provided by the local community, to ensure fair use Gagnef Festival is a 23+ event. You may be required to show ID upon entry.