God is omniscient, that means He knows the only one future. The only one future will become the only one past after some time. Thus we are sure that this future is really only one. It will be identified later. Being free of making decisions means being free to give different directions to the future, but God knows that the only one future before his eyes cannot be changed.
Does this means He isn't free to make any decision?
No. From the context of your question, it means that the decision has already been made.
[Edit: The question is a bit confusing because of the notion of "god" as human with agency vs. the forces of "all that is" which co-informs/creates that agency.]
But let me tell you a little about your theory of GOD called omniscience from a Hebrew's perspective:
The mind of GOD is like all of the trees on planet Earth. Each nodule of every branch is a question posed in his (and her) mind in an epic process of being answered. So, this is, to us, like omniscience in that it is so vast and beyond our human mind.
Yet, S/HE can't ask a question s/he's never thought. This thought doesn't spring infinitely creative, but new thought arises through the interaction and complexity of all S/HE is, including the two humans (who also ate from the Tree of Knowledge which, among other things, confers the ability to be creative).
Therefore, the philosophical notion of omniscience which arose in the Middle Ages (along with omnipotence, omnipresence, etc.) needs to be re-examined and updated. If GOD were omnipotent, for example, we'd have no power at all. These are outdated notions that the Messianic prophesy was designed to answer for Mankind. It is unfortunate and perhaps damnable that postmodernism has thwarted this prophesy and delayed resolution of the world philosophies.
There are alternate views of God's omniscience.
For example, open theists see God's omniscience as knowing everything there is to know, but the free acts of creatures or the free acts of God would not be something that there is to know in advance.
Here is how James Rissler describes it:
Even though God is all-powerful, allowing Him to do everything that can be done, He cannot create round squares or make 2 +2 = 5 or do anything that is logically impossible. Omniscience is understood in a similar manner. God is all-knowing and can know all that can be known, but He cannot know the contingent future, since that too, is impossible. God knows all the possible ways the world might go at any point in time, but He does not know the one way the world will go, so long as some part of what will happen in the future is contingent.
This idea of omniscience would allow for there to be more than one possible future. There may be other ways to conceive of there being more than one possible future.
Let's consider the question: Does this means He [God] isn't free to make any decision?
If omniscience means knowing everything there is to know, but not knowing what is not knowable, such as the precise choices of free agents (including God's own choices), then this would be one way for God or other free agents to make decisions allowing for there to be more than one possible future.
Reference
James Rissler, "Open Theism", Internet Encyclopedia of Philosophy https://www.iep.utm.edu/o-theism/
In traditional Christian theology, God's omniscience means that God knows everything that can be known. There are no gaps in his knowledge. No mysteries to be discovered and nothing to learn. But there are things that can't be known because they're nonsensical. God's omniscience doesn't mean that he knows what a three sided square looks like or how to divide by zero (except when it's defined by convention as in IEEE 754).
In traditional Christian theology God is also said to have free will, though there are more qualifications and disagreements about what exactly that means than for omniscience. In Reformed Protestantism I think God's free sovereign will can be described as God being completely free from any external compelling will, but at the same time that he always wills and acts entirely true to himself. (Most other branches of Christianity would mostly agree with this.) As God's character is that he is loving, truthful, and just, everything he does reflects those aspects of his character. But within the "boundary" of never being inconsistent with who he is (hence his immutability), there is much that he is completely free to choose. For example, there is no inherent reason why the solar system we live in has to have eight planets. Seven planets would have been fine, as would nine or ten. God's decision to create our solar system with eight planets (and thousands of dwarf planets etc.) can accurately be described as a "free" choice.
So how do these two doctrines interact? You're right that on the surface it does seem as if they are in conflict. If God knows all, then that would include all his choices, so wouldn't that mean that to be true to himself there is only one set of choices he can make?
This is one time where we are limited by our nature: we cannot understand how God exists as a timeless being. Even his decision to create the universe was a free choice, for he was under no compulsion to do so, although creating people for him to love is entirely consistent with his character. Having decided to create the universe there were no physical or logical reasons which compelled him to create an eight planet solar system, and its hard to conceive of a reason why that would be part of his character. So I have to conclude that there are many possible universes that he could have created, and that he knows everything about those alternative universes that can be known. The future is what he knows will happen because out of all the possible futures this is the one he has chosen. But I don't think we should think of God's will being constrained by his foreknowledge. It may be better for us to think of God's omniscience and his choices as being "concurrent", although concurrency is not something that applies to a timeless being.
Wherever there is more than one possible reality (such as the number of planets in our solar system), that is proof that God's will is active. As the only self-existing being (the doctrine of Aseity) everything else is contingent on God, and so must depend on him as the ultimate cause, and where there are possible alternatives they must depend on his will.
In these quotes from the Westminster Confession of Faith we can see Reformed Christianity's teachings on God's omniscience, free will, and that he does not will because of his foreknowledge.
God has all life, glory, goodness, blessedness, in and of Himself; and is alone in and unto he himself all-sufficient, not standing in need of any creatures which he has made, nor deriving any glory from them, but only manifesting his own glory in, by, unto, and upon them. He is the alone fountain of all being, of whom, through whom, and to whom are all things; and has most sovereign dominion over them, to do by them, for them, or upon them whatsoever himself pleases. In his sight all things are open and manifest, his knowledge is infinite, infallible, and independent upon the creature, so as nothing is to Him contingent, or uncertain. He is most holy in all his counsels, in all his works, and in all his commands. To Him is due from angels and men, and every other creature, whatsoever worship, service, or obedience he is pleased to require of them. (WCF 2.2)
God from all eternity, did, by the most wise and holy counsel of his own will, freely, and unchangeably ordain whatsoever comes to pass; yet so, as thereby neither is God the author of sin, nor is violence offered to the will of the creatures; nor is the liberty or contingency of second causes taken away, but rather established. (WCF 3.1)
Although God knows whatsoever may or can come to pass upon all supposed conditions; yet has he not decreed anything because he foresaw it as future, or as that which would come to pass upon such conditions. (WCF 3.2)
God the great Creator of all things does uphold, direct, dispose, and govern all creatures, actions, and things, from the greatest even to the least, by his most wise and holy providence, according to his infallible foreknowledge, and the free and immutable counsel of his own will, to the praise of the glory of his wisdom, power, justice, goodness, and mercy. (WCF 5.1)
So according to traditional Reformed Theology, these two doctrines are to be upheld alongside each other, without conflict, and without one being dependent on the other. What is not said is how the timeless God experiences his omniscience and his sovereign will.
Being free of making decisions means being free to give different directions to the future
This is only one view on free will, called libertarian free will.
The other school of thought is compatibilist free will. In this school, free will does not mean that you could have made one choice or the other. Rather, free will means that your actions come directly from within, i.e. from your character, instead of directly from external pressures.
So for example, even though God knows what decisions He is going to make, He is still choosing them freely. That is because being able to do one thing or the other is not a requirement for free will.
For example, a commonly accepted opinion is that God cannot lie, but is still omnipotent. That is because lying is not part of His character and never will be, and so if He could lie, that would mean He didn't have free will.
There is a common thread through all the answers: "There is something we do not know". We don't know that omniscience entails "one past - one future", we don't know how God sees Time or if Knowledge is the same for God as for us. "Free will", "choice", these are anthropocentric concepts. We think in terms of category, duality, causality, agency... There is no justification for applying such ideas to God, whose mind is immutably mysterious to us.
Say "God has free will". Free from what? To be free, there needs be "that which could constrain", to be free from. What does it mean for the Unconstrained to be free? Can we bind God with the mere words of our conclusions, reached by our imperfect attempts at reason? With the known imperfections of Reason itself, does reason even apply in the face of God?
Is reason even needed if God makes a choice? When God chooses there is Being, when not, there IS no choice. God is the source of, the substance of Choice. We choose between alternatives. Our very language doesn't convey the meaning, of our concepts, when applied to God.
We can gain wisdom by contemplating God, we cannot gain knowledge of God by our wisdom
Omniscience implies that the actor (in this case God) knows not only the system upon which he acts (universe, multiverse, world on the back of turtle...) but also himself, fully and completely. Thus, it's not only that such an actor doesn't have a choice when he acts (because choice is not really a choice when you know what you would choose), he cannot really act, because the actor is not temporal, i.e. he doesn't change and the system doesn't change for him (because in effect, he knows every possible state of the system and himself).
We could define relaxed omniscience, where the actor knows the system but he doesn't know himself. In this case, the actor could select certain states of the system (among all possible states), so he does have a certain degree of will. But eventual beings in that system (humans, angels, demons, elves, etc ...) would still not have free will because our actor would know the outcomes of every possible state of the system.
