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 four different types of gammelharpa: the Moraharpa, the kontrabasharpa and the silverbasharpa. You can see two of them both in action on this video. The fourth is a replica of the keyed fiddle found in Siena in Italy, dating 1408. Vicki calls this a Sienaharpa, but that's just Vicki's name for it.
The Moraharpa has an excellent explanation on Wikipedia. So visit that page!