Although we don’t know the name of individuals, we do know several cultures that contributed to the development of Hinduism.
Of these contributing sources, two are specially important for our understanding of the history of the Hindu tradition. The first is the Indus Valley civilization...
How do you know the Bhagavad Gita presents complete Truth? Couldn't it be that some future text will present a more complete truth? You said all authentic scriptures have been revealed according to time and place.