Gentile salvation was not a mystery
The mystery revealed to Paul was not the fact that Gentiles could be saved - this is written large through the prophets, for example…
Isaiah 42:6 (KJV) I the LORD have called thee in righteousness, and will hold thine hand, and will keep thee, and give thee for a covenant of the people, for a light of the Gentiles;
…and can be traced back via Genesis 12:3 to Genesis 3:15. The mystery was HOW they can be saved.
Under the prophecy programme, Gentiles are saved at the restoration of Israel:
Isaiah 49:6 (KJV) 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.
In the dispensation of grace, under the mystery programme, Gentiles are saved without Israel:
Ephesians 3:6 (KJV) That the Gentiles should be fellowheirs, and of the same body, and partakers of his promise in Christ by the gospel:
Fellowheirs - not secondary
Of the same body - wqual standing (not graffed in)
Partakers of his promise…by the gospel - not through Israel’s law or covenants.