The Bible does not confirm precisely what He was taught. Paul only tells us that Abraham was taught the Gospel. But because Paul states that the Gospel was kept hidden, by God, as a Mystery . . . I would have to say that the details were kept from us all. But to say that Abraham was taught the Gospel, I would have to conclude that he understood is according to the following details:
The Circumcision of the Messiah
The granting of Repentance from the Messiah
The sending and free Gift of the Holy Spirit by the Messiah
The future death, burial, and Power of the Resurrection of the Messiah that would eventually pay the debt for his sin, thus making himself, Abraham, Right with God
Abraham was taught the Purpose, Work, and Effect of the Messiah . . . a changed Heart.