Therefore, as it is stated in the Śrīmad-Bhāgavatam, the Absolute Truth is realized in three phases, vadanti tat tattva-vidas tattvaṁ yaj jñānam advayam (SB 1.2.11). Advayam means nonduality, one. The one supreme truth, Absolute Truth, is realized in three phases, brahmeti paramātmeti bhagavān iti śabdyate: realization of the impersonal Brahman, or the glowing effulgence, just like sunshine, then the localized Supersoul, then Bhagavān, the Supreme Personality of Godhead.
And there are many expansions of the Supreme Personality of Godhead: Nārāyaṇa, Adhokṣaja... There are many innumerable planets in the spiritual world, and all of them are emanation from the Kṛṣṇa planet. And the Kṛṣṇa planet, the supreme deity is Kṛṣṇa.
This description we have got in several Vedic literatures, Brahma-saṁhitā and Vedānta-sūtra, Bhāgavata, in Purāṇas, in Upaniṣad. Everywhere these descriptions are there. Those who are scholarly student, they'll get information, and the whole thing is summarized in Bhagavad-gītā. Therefore Bhagavad-gītā is known also as Gītopaniṣad. At the end of every chapter you'll find these things are written, gītāsu or upaniṣatsu. It is one of the Upaniṣads. You have read Upaniṣad. The Bhagavad-gītā is also one of the Upaniṣads, and therefore Bhagavad-gītā is known as Gītopaniṣad.