Complex sentence: Nandvanshi is a term designating the claimed descendants of Nanda (or "Nandagopa"), who according to the Harivamsha and the Puranas, was the head of the Gopas (a tribe of cowherds referred as "Holy Gwals").
Split into simpler sentences: Nandvanshi is a term designating the claimed descendants of Nanda (also known as Nandagopa). According to the "Harivamsha" and the "Puranas", Nanda was the head of the "Gopa" s (a tribe of cowherds referred as "Holy Gwals").