How could the Gentiles in unbelief know to call upon the Lord to be saved? Until the Gospel of Christ was sent unto them, they could not believe in whom they did not know. They learned of Christ through His Gospel preached unto them. Before the advent of Christ only Israel had heard of the gospel of good tidings to come, but many/most of them did not believe, and died in unbelief. That was the fate of Gentile nations before the advent of Christ sending the Gospel unto all the nations of the world.
Romans 10:13-21 (KJV) For whosoever shall call upon the name of the Lord shall be saved. How then shall they call on him in whom they have not believed? and how shall they believe in him of whom they have not heard? and how shall they hear without a preacher? And how shall they preach, except they be sent? as it is written, How beautiful are the feet of them that preach the gospel of peace, and bring glad tidings of good things! But they have not all obeyed the gospel. For Esaias saith, Lord, who hath believed our report? So then faith cometh by hearing, and hearing by the word of God. But I say, Have they not heard? Yes verily, their sound went into all the earth, and their words unto the ends of the world. But I say, Did not Israel know? First Moses saith, I will provoke you to jealousy by them that are no people, and by a foolish nation I will anger you. But Esaias is very bold, and saith, I was found of them that sought me not; I was made manifest unto them that asked not after me. But to Israel he saith, All day long I have stretched forth my hands unto a disobedient and gainsaying people.