“In the beginning of the Tretā-yuga [millennium] this science of the relationship with the Supreme was delivered by Vivasvān to Manu. Manu, being the father of mankind, gave it to his son Mahārāja Ikṣvāku, the King of this earth planet and forefather of the Raghu dynasty in which Lord Rāmacandra appeared. Therefore, Bhagavad-gītā existed in the human society from the time of Mahārāja Ikṣvāku.”
At the present moment we have just passed through five thousand years of the Kali-yuga, which lasts 432,000 years. Before this there was Dvāpara-yuga (800,000 years), and before that there was Tretā-yuga(1,200,000 years). Thus, some 2,005,000 years ago, Manu spoke the Bhagavad-gītā to his disciple and son Mahārāja lkṣvāku, the King of this planet earth. The age of the current Manu is calculated to last some 305,300,000 years, of which 120,400,000 have passed. Accepting that before the birth of Manu, the Gītāwas spoken by the Lord to His disciple, the sun-god Vivasvān, a rough estimate is that the Gītā was spoken at least 120,400,000 years ago; and in human society it has been extant for two million years. It was respoken by the Lord again to Arjuna about five thousand years ago. That is the rough estimate of the history of the Gīta, according to the Gītā itself and according to the version of the speaker, Lord Śrī Kṛṣṇa. It was spoken to the sun-god Vivasvān because he is also a kṣatriya and is the father of all kṣatriyas who are descendants of the sun-god, or the sūrya-vaṁśa kṣatriyas. Because Bhagavad-gītā is as good as the Vedas, being spoken by the Supreme Personality of Godhead, this knowledge is apauruṣeya, superhuman. Since the Vedic instructions are accepted as they are, without human interpretation, the Gītā must therefore be accepted without mundane interpretation. The mundane wranglers may speculate on the Gītā in their own ways, but that is not Bhagavad-gītā as it is. Therefore, Bhagavad-gītā has to be accepted as it is, from the disciplic succession, and it is described herein that the Lord spoke to the sun-god, the sun-god spoke to his son Manu, and Manu spoke to his son Ikṣvāku.
Ref >> Srila Prabhupada Vani.