Hinduism was not a religion founded by any one person.
It was the way of life of the people living in the Indian subcontinent, across the 'Sindh' river. At that time it was known as "Sanatan Dharma" or "Eternal Religion".
The teachings of Hinduism were passed down from generation to generation in the form of Vedas.(oral transmission). These Vedas contain knowledge and teachings which is in the purest form and directly revealed by the Almighty to enlightened "rishis".
Bhagavad Gita (Song of the God) is the holy book of Hindus. However most of the philosophy in Bhagavad Gita is derived from the Vedas (Rig Veda, Sama Veda, Yajur Veda and Atharva Veda) which act as the foundation stone of Hindu beliefs.
To add to it, Bhagavad Gita is a part of the larger epic "Mahabharata"