Did the Old Testament teach that Gentiles would become one with God?
No. Scripture says the mystery of Christ was not made known in other generations, so the Old Testament did not reveal that Gentiles would become fellow heirs, one body, and partakers of the promise in Christ. That truth was hidden until Paul's revelation, not taught by Judaism or the Mosaic law.
Les teaches that Ephesians 3:5-6 says the mystery of Christ was not made known in other ages or past generations—taking us all the way back to the Old Testament. The specific segment of the mystery here is that Gentiles should be fellow heirs, of the same body, and partakers of His promise in Christ, not by Judaism or the Mosaic law, but by Christ's finished work. Les points out that nothing like this was read in the Old Testament, that Gentiles would become one with Christ or with God, or one with the God of Abraham. They did not understand Christ as much back then, and such a thing was unheard of until it was revealed to Paul.