Just to transfer their thinking power to the spiritual sky, Kṛṣṇa-dvaipāyana Vyāsa has given a great number of Vedic literatures. First he divided the Vedas into four, then he explained them in the Purāṇas, and for less capable people he wrote the Mahābhārata. In the Mahābhārata there is given the Bhagavad-gītā. Then all Vedic literature is summarized in the Vedānta-sūtra, and for future guidance he gave a natural commentation on the Vedānta-sūtra, called Śrīmad-Bhāgavatam.
Mahabharata (BG and SB)
Bhagavad-gita As It Is
BG Preface and Introduction
BG Introduction:
"One who drinks the water of the Ganges attains salvation, so what to speak of one who drinks the nectar of Bhagavad-gītā? Bhagavad-gītā is the essential nectar of the Mahābhārata, and it is spoken by Lord Kṛṣṇa Himself, the original Viṣṇu." (Gītā-māhātmya 5)
BG Chapters 1 - 6
BG 1.1, Purport:
The topics discussed by Dhṛtarāṣṭra and Sañjaya, as described in the Mahābhārata, form the basic principle for this great philosophy. It is understood that this philosophy evolved on the Battlefield of Kurukṣetra, which is a sacred place of pilgrimage from the immemorial time of the Vedic age. It was spoken by the Lord when He was present personally on this planet for the guidance of mankind.