Of the unreal there is no being; the real has no nonexistence. But the nature of both of these, indeed, has been realized by the seers of Truth.
Here Krishna is expressing the Samkhya doctrine of satkaryavada and it is a central element in the in the ontology of the Gita. It is also expressed as the non-difference of cause and effect. It is not a syncretic strain to see it as very like the thought of Parmenides : cf.
In the Gita the satkaryavada thesis is used to subvert the feeling of aham karta (I am the doer). We act thinking we are achieving our goals when in fact the goals are already there as potency. This is expressed as the non-difference of cause and effect. What is already there emerges. If it was not there it could have no traction on reality in order to emerge; a vertiginous theory that leaves the actor falling into an abyss of non-being.
This certainly seems like determinism and that is a defensible interpretation looked at solely from a cosmic perspective. It could also be viewed as the general context or background against which agential libertarian activity takes place. Everything arises out of causes and conditions and in retrospect we can give a causal account, prospectively we have options. Freedom isn't free, it's a duck rabbit kind of thing.
There is also the doctrine from the Upanishads, particularly the Chandogya Up,that Brahman is the material cause of the universe. The classic analogy of material identity, clay and vessels of clay and the claim that their fundamental reality is just clay. Only name and form changes. I can see where this is useful as an analogy but when it is proposed as an ontology with the claim that cups and pots are unreal and only the clay is real then I think that there is a confusion of different levels of discourse. There are cups made of clay and there are plates of clay. At the level of material reality they are just as real as clay itself. The real/unreal ontological discussion is about the general absorption into a higher unchanging reality of the material. An interim holding position is sometimes taken as final. My sense of the Indian tradition is that they give you just as much of the truth as you can handle until you are ready for an update. Because the teaching template is a guru/sisha one the confusion created in the mind of the simple occidental by contrary messages is not a factor. The groves of the academy are still the groves of the Academy with plato and aristotle delivering their evolved thought to everyone at the same time.