You will have been told by those anxious to place Advaita in recognisable categories that it is essentially pantheistic, monistic, panentheistic, or indeed, in the popular formula - All is One. The relation between the Creator and the Created is a subtle one in the Bhagavad Gita. Two citations in Chapter 9 are apposite:
B.G. 9:4
This whole world is pervaded by Me in My unmanifest form. All beings exist in Me, but I am not contained in them!
B.G. 9:5
5. Nor do the beings dwell in Me. Behold My divine Yoga! I am the sustainer and originator of beings, but My Self is not contained in the beings.
What can be taken from these two sutras is the contingent nature of creation. It might not have been, but it is. Contingent being requires something more than itself to continue in existence. That must be a necessary being which sustains by its nature the contingent. Obviously contingent beings are not one with the necessary. They are in it by virtue of being but not of it.
In Sankara''s commentary he states that being a thing means being a something distinct from other things.
“Therefore, being possessed of their reality through Me, who am their Self, they exist in me. Hence they are said to be established in Me.”
Obviously these sutras rule out pantheism or the notion that God and the universe are identical. Panentheism or the idea that out of the nature of God the universe arose in a quasi necessary way must also fail to pass the advaitic test..
Gaudapada with his ajativada (unborn) doctrine would probably say that contingent being is non-being and that only being exists. What does not have being of its nature i.e. necessary being, cannot come to pass. Somewhat annihilating!