None of these verses say that the 4 gospels belong to the Old Testament and we should pay attention only to the church epistles.
Nor do they say the gospel proclaimed to the gentiles was different from the gospel proclaimed to the Jews.
Acts 9:15
But the Lord said to Ananias, Go! This man is my chosen instrument to proclaim my name to the Gentiles and their kings and to the people of Israel.
NOTE: Paul was a chosen instrument, but not the only chosen instrument. Paul was just one among many who spread the gospel to gentiles.
Peter preached to Cornelius, a Roman centurion of the Italian Regiment stationed in Caesarea.
Phillip preached to the Ethiopian eunuch.
Romans 11:13,14
I am talking to you Gentiles. Inasmuch as I am the apostle to the Gentiles, I take pride in my ministry
in the hope that I may somehow arouse my own people to envy and save some of them.
NOTE: Paul was preaching to gentiles, but also to Jews.
Romans 15:16
That I should be the minister of Jesus Christ to the Gentiles, ministering the gospel of God, that the offering up of the Gentiles might be acceptable, being sanctified by the Holy Ghost.
NOTE: This does not say that nobody else was ministering the gospel to gentiles.
Phillip preached to the Ethiopian eunuch.
Was that BEFORE Gal 2:7-9 or after ? :)

