The first time I’ve heard about that term it was prounced by “ASH” as the name of a cool algerian folk group. Typing that word in google brings much bright ness , in fact Gnawa is a 600 years music old :

Gnawa is a term that has two important meanings. It is used to define both a religious/spiritual order of a traditionally Moroccan Black Muslim group and a music style connected to this order. The term encompasses all members of the Gnawa: the master musicians to the players of the Karkaba (metallic castanets) to the disciples and women soothsayers/therapists.
Gnaw defintion hit in conflict with what used to know -here in Tunisia- Bou sahdia (بوسعدية), the gnawa should be more significant. Gnawa is know as “The blues of morocco”.

In the other hand Gnawa diffusion is the exact name of a gathering of young peapole from Algeria playaing the Gnwa music who have mixed folk and modern instuments they have started since 1992 at Grenouble in France, then their first album with 5 titles to detonate an explosive success : Tv-Shows, international concerts….
Gnwa defintion [En]
Gnwa Diffucion offical Site [Fr]
Gnwa Fans site [Fn]