Unlike the non-Vedic scriptures (scriptures from outside India), the Bhagavad-Gita, is not a one off scripture, it’s eternal. It was last spoken 5000 years ago by Lord Krishna to Arjuna, and 120.533 million years ago to the person who maintains the sun planet, Vivasvan. It was first spoken in this universe, 155.522 trillion years ago. The Bhagavad-Gita has also been spoken by Lord Krishna in millions of other universes.
“I gave this science of self-realization knowledge to Vivasvan, and he gave it to Manu, and Manu gave the knowledge to his son, Iksvaku.”(Lord Krishna, Bhagavad-Gita 4.1)
Vivasvan is a demigod who maintains the sun planet. Demigods are very powerful human beings, they are NOT the Supreme (the God).
“Although I am unborn, imperishable, eternal, and the Lord of all living beings, using My internal energy, I still appear in My original spiritual form.”(Lord Krishna, Bhagavad-Gita 4.6)
Lord Krishna appears once on this planet in His original form, once in every daytime of Brahma, which is every 8.64 billion years, and each time the Bhagavad-Gita is spoken by Him. So far in the current cosmic cycle, Lord Krishna has appeared and spoken the Bhagavad-Gita more than 18,000 times on this planet (age of universe/timing of appearance=155.522 trillion/8.64 billion). This is how great Lord Krishna (God), His teachings, and the Bhagavad-Gita are.
“Once in a day of Brahma, He descends to this world to manifest His transcendental pastimes.”(Caitanya-caritamrta Adi-Lila 3.6)