Nobody knows exactly how far back the roots of Indonesia's national music style go. But certainly, gamelan must date back to at least the 8th century CE.
This beautiful bas relief carving is from the Borobudur temple complex in central Java. It shows court musicians from the Srivijaya Empire era playing many of the same instruments that can be found in gamelan orchestras to this day.
Photo by Gunkarta via Wikimedia.