Like Ved, the Gita is eternal and is reproduced on the earth planet at regular intervals. When Shree Krishna told the Gita to Arjun 5,000 years ago, it was not the first time it was told. In the beginning of the fourth chapter, Shree Krishna tells Arjun that He had revealed the same knowledge to the sun god Vivaswat before the beginning of the current manvantar (a period of time corresponding to one hour in Brahma’s day; there are 14 manvantar in one day of Brahma – we are currently in the seventh). Before that, there were many other times as well, because Shree Krishna tells the same Gita every time He descends on earth.
The Gita is part of the extensive and fully coordinated system of Sanskrit scriptures that forms the basis of Sanatan Dharm, the eternal and universal religion for all the souls. The word ‘sanatan’ means eternal, and ‘dharm’ means those thoughts, words and actions that foster good qualities in a person, and ultimately take him to God. So, if Sanatan Dharm is eternal, then it means the knowledge of Sanatan Dharm is eternal. That knowledge remains in the form of the Ved, Upnishad, Gita and other Hindu scriptures, which reside eternally within God in their Divine form and are maintained here on the earth planet in a book form. It is all a part of the process of srishti: the manifesting and maintenance of the creation which gives a chance for the souls to realize God.