What is the difference between Atma gayan(the knowledge of Self) and the the knowledge of upanishad.
Knowledge of the Upanishads is intellectual knowledge. Atmajnana which is the same as Brahmajnana is anubhuti (direct experience) of Brahman and not simply intellectual knowledge. They are different.
Brahmajnana or Atmajnana
The Vedanta system of philosophy posits Brahman as the origin of the entire universe including the inanimate nature and the living beings. The universe evolves out of it, it is sustained in it and involves back to it. Its essential nature is 'sat-cit-ananda', existence-consciousness-bliss. It is uncaused and eternal. It is also the very essence of the human beings. Since the human beings do not know this Brahman that is in them or behind them, they are undergoing a lot of suffering caused by the vicious circle of birth-death-rebirth, technically called 'samsara'. Moksha or liberating oneself from the bondage of this samsara, is the ultimate goal of life. This moksha can only be achieved by brahmajnana.
Though the word 'jnana' means 'knowledge, 'brahmajnana' does not mean an intellectual understanding of Brahman. It is anubhuti or direct experience of Brahman, not only as the substratum of the entire creation, but also as the innermost core of oneself. Once this direct experience comes, there will no more be any identification with the body-mind complex which alone was responsible for samsara.
A concise encylopaedia of Hinduism by Swami Harshananda