Did Abraham understand the gospel that was preached to him?
No. The Gospel preached to Abraham was not a Gospel that Abraham understood. He only had a concept that God was going to send a Redeemer. The mystery was kept secret in God's mind and not revealed to Old Testament saints.
Les explains that the Gospel preached to Abraham was not a Gospel Abraham understood. The Old Testament saints could not know it because it was a secret kept in the mind of God, and God did not reveal it to them. Even the Twelve did not understand that Jesus would die and rise again; they thought it was all over, and Peter went fishing. Les notes that today many are kept blind to the truth of the Gospel until God opens their eyes; then they become intensely responsible by His sovereign grace. Abraham only had a concept that God was going to send a Redeemer, and Israel still looks for the Day of Redemption.