The Vedas, Upanishads and other texts of the ancient seers were vast storehouses of observation, information and analysis, along with practical guidelines for gaining an understanding through personal experience. They in fact started from the experience, observation and standpoint of a specific seer in many cases. While they focused on and moved toward the central unifying experience, they did so from multiple unique and individual views, and thus, came at the observation and the solution in multiple different ways. They were, therefore, a fragmented view of the unity, and more or less represented the attempt of the blind men to describe the elephant by each one touching a different part of it, and thereby coming up with vastly different descriptions.
The Gita, on the other hand, is attempting to unify these different descriptions into a unified whole. The standpoint can be compared to the concept of the first time humans observed the planet earth from outer space, and began to recognise that the world is one unified eco-sphere and bio-sphere and that all human beings are one species and just one part of this unified biological and ecological whole.
The Gita therefore does not outright dismiss any particular path or philosophical direction that was current at the time, but works to integrate it into a more complete web of understanding, eliminating thereby the most exclusive and rigid aspects of each one, and looking to find where it fits into the larger picture and thereby harmonises with the others.
During this process, the Gita adds its own contributions which provide some of the unifying factors. Sri Aurobindo describes the process of the Gita: “The Gita has to synthesize the Yoga doctrine of liberation by works and the Sankhya doctrine of liberation by knowledge; it has to fuse karma with jnana. It has at the same time to synthesize the Purusha and Prakriti idea common to Sankhya and Yoga with the Brahmavada of the current Vedanta in which the Purusha, Deva, Ishwara,–supreme Soul, God, Lord,–of the Upanishads all became merged in the one all-swallowing concept of the immutable Brahman; and it has to bring out again from its overshadowing by that concept but not with any denial of it the Yoga idea of the Lord or Ishwara. It has too its own luminous thought to add, the crown of its synthetic system, the doctrine of the Purushottama and of the triple Purusha for which, though the idea is there, no precise and indisputable authority can be easily found in the Upanishads and which seems indeed at first sight to be in contradiction with that text of the Shruti where only two Purushas are recognised.”
Other views are also harmonised along the way as the Gita addresses the main lines of Sankhya, Yoga and Vedanta in its integrating view.
Sri Aurobindo, Essays on the Gita, First Series, Chapter 9, Sankhya, Yoga and Vedanta, pp. 82-83,