In this answer, it is told that Gandharva Veda (Music Theory) is one of the four Upa Vedas, the other three being Ayurveda, Dhanurveda and Arthashastra.
What is the oldest text on Gandharva Veda? Was Gandharva Veda written/conveyed by Gandharvs, like Ayurveda was conveyed by Lord Dhanvantari?