These are some of the books and articles that I would recommend you to read if you want to learn more about sound healing in general and the gongs in particular. The list starts with Sheila Whittaker because her books are so informative and is then alphabetic.
Whittaker, Sheila – Sound Healing with Gongs – a Gong Book for Beginners, 2010
Whittaker, Sheila – In the Heart of the Gong Space – the Gong as a Spiritual Tool, 2012
Brahma, Nada – The World of Sound, 1991
Gaynor, Michael L. – The Healing Power of Sound, 1999
Goldman, Joshua/Sims, Alec W – Sound Healing for Beginners, 2018
McKusick, Eileen Day – Tuning the Human Biofield: Healing with Vibrational Sound Therapy, 2014
Pesek, A and Bratina,T – Gong and its Therapeutic Meaning – October 2016
Scallon, L – The Healing Power of Sound, New Dawn, September-October 2006
There are also relevant articles that you can locate and download on the internet:
Franklin, Ellen, From Galaxies to Cells in Subtle Energies & Energy Medecine, vol.16, no. 3 pp. 283-317
Gaumond, D, The Power of Mantra in International Journal of Healing and Caring, Volume 7, no.2, May 2007
Simon, What is Sound Healing? in International Journal of Healing and Caring, Volume 7, no.3, Sept. 2007