Most of their repertoire is the acoustic folk music of these Swedish-speaking Finns, from the unique minuets and ballads that have only survived in Ostrobothnia, to the old traditional waltzes.

The didgeridoo and sub-contrabass recorder offer an underlying drone, a technique shared by some other Nordic bands such as Garmarna.

Also notable is their use of the hardanger fiddle and Jenny Wilhelms' kulning, a high-pitched, wordless vocal technique based on traditional Scandinavian cattle-herding calls.

Gjallarhorn was formed in 1994 on the west coast of Finland, in the Swedish speaking area, by Jenny Wilhelms, Christopher Öhman (viola, mandola) and Jacob Frankenhaeuser (didgeridoo).

In 1998 the band started touring frequently on a professional basis with performances at international world, folk and jazz music festivals.