So also we, while we were children, were held in bondage under the elemental things of the world. 4 But when the fullness of the time came, God sent forth His Son, born of a woman, born under the Law, 5 so that He might redeem those who were under the Law, that we might receive the adoption as sons. (Galatians 4:3-5)
So we see that, again, Gentile believers are adopted into the New Covenant, which God made with the Jews, not Gentiles, and we are participants, not takers-over. God specifically singles out the "children of promise", who are descendants of the son of Sarah and who come to faith in Jesus Christ, as the true Israel.
Where do you find Gentile believers in this passage?
“THROUGH ISAAC YOUR DESCENDANTS WILL BE NAMED.” 8That is, it is not the children of the flesh who are children of God, but the children of the promise are regarded as descendants. 9For this is the word of promise: “AT THIS TIME I WILL COME, AND SARAH SHALL HAVE A SON.”
Was Sarah, who gave birth to the child of promise, a Jew or Gentile? Was her son, Isaac, a Jew or a Gentile? Who did God use to create those who He calls the "children of promise" Was it a Jew or a Gentile?
6* Now to Abraham and his seed were the promises made. He saith not, And to seeds, as of many; but as of one, And to thy seed, which is Christ. 17* And this I say, that the covenant, that was confirmed before of God in Christ, the law, which was four hundred and thirty years after, cannot disannul, that it should make the promise of none effect. 18* For if the inheritance be of the law, it is no more of promise: but God gave it to Abraham by promise. 19* ¶ Wherefore then serveth the law? It was added because of transgressions, till the seed should come to whom the promise was made; and it was ordained by angels in the hand of a mediator. 20* Now a mediator is not a mediator of one, but God is one. 21* Is the law then against the promises of God? God forbid: for if there had been a law given which could have given life, verily righteousness should have been by the law. 22* But the scripture hath concluded all under sin, that the promise by faith of Jesus Christ might be given to them that believe. (Galatians 3:16-22)
It has NOTHING to do with fleshly descent. All are concluded under sin. The promise is available to ALL that believe on the LORD Jesus Christ.