The legend of Gangotri goes to a long time back when King Sangara had left a horse to run as far as it could uninterrupted as a part of a ritual. It was said that whatever territory the horse would run on that land will come under the King and if someone refuses to obey then they would have to face an army of his 60,000 sons. Lord Indra had tied the horse near Sage Kapil who was deep in meditation and when the sons came to demand the land, they interrupted his meditation, which made the Sage angry and killed all of the sons at once. Hence, Goddess Ganga had to be brought down for putting the souls of those dead men to rest and peace.
Suraj Kund is said to be where Goddess Parvati used to take her bath and hence people pay homage to the Sun God there.