The term gammelharpa really just means old-harpa and is a term that covers anything that isn't a modern three-row chromatic nyckelharpa. Vicki plays two different types of gammelharpa: the Moraharpa and the kontrabasharpa. You can see them both in action on this video:
The Moraharpa has an excellent explanation on Wikipedia. So visit that page!
Vicki is building an alspel list on both - watch this space.