All your quotes are conjectures. John 6:38 does not say anything about Gentiles. The OT says nothing about evangelizing the Gentiles. Those who'd see Him and believe in Him would become Jewish.
No mention was made of a "new" religion. The earliest of Chirstians considered themselves Jewish and attended synagogue services until they were literally kicked out of them and out of Israel.
It's only at this point that Christianity begins to change form being a sect of Judaism, a denomination if you will, like the Sadducees and Pharisees and Essenes, to a different religion.
Of course, God forknew all of this ... and so, it can be concluded, ... always intended to extend His salvation to the Gentiles.
The Old Testament scriptures do affirm this ...Isaiah 49:5 And now, saith the LORD that formed me from the womb to be his servant, to bring Jacob again to him, Though Israel be not gathered, yet shall I be glorious in the eyes of the LORD, and my God shall be my strength.And, of course, Jesus was/is God, ... so obviously Jesus was aware and approving of the gospel being preached to the Gentiles at the appointed time.
6 And he said, It is a light thing that thou shouldest be my servant to raise up the tribes of Jacob, and to restore the preserved of Israel: I will also give thee for a light to the Gentiles, that thou mayest be my salvation unto the end of the earth.
I am sure He foreknew it; He is just not quoted as saying anything about it.
Your example of 49:5 can be countered by other examples form the OT where even the unbelieving Hebrews are referred to as "Gentiles" (Gen 12:2 and 25:23). There is nothing specific about the word Gentile (a Latin word meaning "clan" or "tribe." The same is true of the Greek equivalent ethnos, and Hebrew "Goy."
It is in the NT that we see more famliar usage of the word Gentile to denote a non-Jewish (pagan) individual. This is further accentuated in the KJV version of the Bible.