The Gita incorporates, in the first six chapters, most of the premises of the Vedantic concept of knowledge. Sri Aurobindo describes it: “The Gita takes it over at once and completely and throughout the six chapters quietly substitutes the still immutable Brahman of the Vedantins, the One without a second immanent in all cosmos, for the still immutable but multiple Purusha of the Sankhyas. It accepts throughout these chapters knowledge and realisation of the Brahman as the most important, the indispensable means of liberation, even while it insists on desireless works as an essential part of knowledge.”
The Gita weaves together the concepts of Sankhya and Vedanta in a way that takes up, expands and harmonises their positions. In the way of knowledge, however, it is clear that the Vedantic viewpoint is predominant.
At the same time, both Sankhya and Vedanta, in the sense that they were understood and practiced at the time, were unable to resolve the ultimate question that would be able to integrate action and inaction, the immobile and the mobile. The Gita addresses this by describing a supreme Purusha, Purushottama, which was able to hold within itself, without conflict or disharmony, both the unmoving and the moving concurrently.
“The Brahman is its supreme and not in any lower aspect has to be presented as the Purusha with the lower Prakriti for its Maya, so to synthesize thoroughly Vedanta and Sankhya, and as Ishwara, so to synthesize thoroughly both with Yoga; but the Gita is going to represent the Ishwara, the Purushottama, as higher even than the still and immutable Brahman, and the loss of ego in the impersonal comes in at the beginning as only a great initial and necessary step towards union with the Purushottama. For the Purushottama is the supreme Brahman.”
The concept is alluded to in the Upanishads, but it is the Gita which clearly sets it forth and positions it in such a way as to allow all the major schools of understanding to fit within it.
Sri Aurobindo, Essays on the Gita, First Series, Chapter 9, Sankhya, Yoga and Vedanta, pp. 84-85,