Was there any concept of Gentiles being brought into a relationship with God in the Old Testament?
No. The Old Testament prophetic program was Jew only, and there was no concept of Gentiles being brought into a relationship with the Almighty God. That door opened later through the revelation of the mysteries, during the Age of Grace, when Paul's ministry among the Gentiles began.
Les explains that the Old Testament prophetic program was "Jew only" and that prophecy had run its course when Christ came, was rejected, and crucified. With Israel's continued rejection, that program came to a screeching halt, God's time clock stopped, and He opened a new vista of Scripture: the mysteries. Les points to Romans 16:25 and Paul's gospel, the preaching of Jesus Christ according to the revelation of the mystery. For almost two thousand years, Les says, we are in the Age of Grace, not the prophetic program. These were things kept secret in the Old Testament, so there was no concept of Gentiles being brought into a relationship with the Almighty God.