Originally from the sub-Saharan ("Sahel") regions of Africa, notably Senegal, the Gwana are ritual musicians who originally came to Morocco as slaves in the 15th and 16th century from the vast Songhai Kingdom that existed across the Sahel region at the time.
Their musical skill was known as being able to heal 'possessed' individuals through their music that induces a state of trance.
There is annual music festival in Essaouira, 2015 now in its 18th year, has become a fixture on the world music festival calendar. Focusing on the traditional Gnaoua (Gnawa) music, it also includes jazz, pop, rock, and contemporary world music by musicians from all over the world.
Further interesting reading is to be found in Deborah Kapchan's article at: