I am not sure I entirely understand your question. The Bhagavad-Gita was revealed by Lord Krishna and it was Sage Vyasa that was spiritually one with Krishna who helped compose the text and give it a written form. Why is it so difficult to understand. In any case spiritual or physical laws do not have to be in written form to exist. The law of gravity existed and operated before it was discovered and put down in a written form. Before God created the physical cosmos, He came up with a blue print which is a matrix of ideas , principles and laws out of which came the cosmos. The blue print still exists in the ether and prophets can tap into the blue print and reveal God's laws hidden from ordinary people when it is necessary and appropriate in accordance with the will of God. You should read Bahaullah's mystical poems and other writings like Gems of Divine mysteries to understand his insight in to esoteric spiritual laws. Likewise you need to read the Hindu scriptures. Simply dismissing them as myths is your ignorance and perhaps spiritual arrogance.How can you write something down without scripture? That is totally nonsense. I think you should check your resources and stop repeating Hindu myths.