Gangotri is a small town & one of the Char Dham Sites located in Uttarakhand. Also known as an abode of Ganga. The word Gangotri is a combination of two words that describe the event i.e. Ganga + Uttari, which roughly translates into Ganga descending.
According to Valmiki Ramayana १-४३-२६-
तत्र ऋषि गण गन्धर्वा वसुधा तल वासिनः
भव अंग पतितम् तोयम् पवित्रम् इति पस्पृशुः |
And asserting that the water as holy, because it descended touching the head of Shiva, the assemblages of sages, gandharva-s, and those that are residents on the plane of earth have sipped that water at that place.
According to Mahabharata- Vana Parva- Tirth Yatra Parva- Chapter 85 - Verse 97 -
यत्र गंगा महाराज स देशस्तत तपोवनम |
सिद्धक्षेत्रं च ताज्ञ्नेयं गंगातीरसमाश्रीतम ||
O great king, the region through which the Ganga flows should be regarded as a sacred asylum, and a spot of land that is on the Ganga's banks, should be regarded as one favourable to the attainment of ascetic success.
The origin of the ganga is at Gaumukh, set in the Gangotri glacier which is 19 km trek from Gangotri. But according to our scriptures, Gangotri is the place where river Ganga descended from heaven when Lord Shiva released the mighty river from his luscious locks (jatta).
Actually, King Sagar's grandson Anshuman remained unsuccessful to bring Ganga on earth and same was with his son Dilip. However, King Anshuman's grandson Bhagirath was very resolute for this task and meditated at Gangotri. After numerous years of meditation, Ganga came down from the paradise but remained in suspense in the coils of Lord Shiva's hair. Determined to bring Ganga, King Bhagirath started meditation in order to win Lord Shiva. Being pleased with it, Lord Shiva released the Ganga as three streams.
One of these streams came on the earth in the form of Bhagirathi.