Sangeet is first mentioned and it's origin/usage described in the monumental arts treatise written in the late Vedic Era (over 2,000 years ago) called Natya Shastra.
Sangeet is described as the 'celestial song' which is sung in Ghandarva (a word repeatedly mentioned in Mahabharata) - the realm of the Gods. Brahma Ji is said to have given this gift to teh great sage Narada - who was the first human to receive this divine gift. His objective was given - to spread the gift to humanity to ease our suffering and create the practice of divine worship in it's highest form.
So we see that the word sangeet, which today has more art/entertainment based meaning, was originally used to mean divine song - and thus sung in the house of God (Mandhirs).
Today in India we have 3 main forms of sangeet - Hindustani sangeet (North Indian - which is a fusion of Indic and Persian traditions - to which Gurmat Sangeet belongs), Carnatak Sangeet (South Indian - a more pure form of ancient Indic Sangeet as it has not been tampered by Islamic or British influence/rule) and Rabindra Sangeet - created by the genius, poet, artist, musician, novelist and mystic (1st ever Asian winner of the Noble Prize) Rabindranath Tagore - who fused ancient Sufi Bengal folk styles with traditional Indic Sangeet.