The Gospel preached to Abraham was... The Gospel of Jesus Christ, the same Gospel those in Christ Jesus have believed today.
You say that the same gospel Paul preaches below was preached to Abraham:
1 Corinthians 15:3 For I delivered unto you first of all that which I also received, how that Christ died for our sins according to the scriptures;
15:4 And that he was buried, and that he rose again the third day according to the scriptures:
The Bible says the gospel preached to Abraham was "In thee shall all nations be blessed".
Galatians 3:8 And the scripture, foreseeing that God would justify the heathen through faith, preached before the gospel unto Abraham, saying, In thee shall all nations be blessed.
These are not the same gospel! They are not the same words, not the same content, and therefore are different!
What was revealed to Abraham was only a shadow of the whole gospel, what God foresaw, that the Gentiles would be justified through faith in Christ, was not explicitly stated. What was revealed to Abraham was to be believed.
You don't know this because you have not paid attention in Scripture, but instead have been fed men's doctrines that get totally away from this Truth in Scripture.
At the end of John 8, Jesus said Abraham saw His day, and was glad. The Pharisees mocked Him, and asked Him if He had seen Abraham. Then Jesus said, "Verily, verily, I say unto you, Before Abraham was, I AM." (John 8:58) Then the Pharisees picked up stones to try and stone Him, because by that Jesus proclaimed that He is God.
In Genesis 12:1-3 Abraham was commanded to go unto a land God would show unto him and be made a great nation which would be a blessing. In Hebrews 11:8-10 Abraham was looking for a city built by God. Jesus came to confirm the promises made unto the fathers (Romans 15:8) and to establish the Davidic kingdom to Israel; IN Mark 1:14-15 Jesus preached the gospel of the kingdom of God and said the time is fulfilled; this was the day Abraham was looking for. This was the day promised when Israel would be a holy nation and be a blessing to the Gentiles in the kingdom.
The deeper point is that Jesus in the Old Testament did meet Abraham, and offered Abraham bread and wine, and blessed him. Jesus was the Melchizedek of Genesis who met Abraham. Hebrews 7 reveals this.
Hebrews 7:3 says Melchisedec was made like unto the Son of God. He is not Christ.
And then in Genesis 18, Abraham at his tent door, looks up and all of a sudden there are three men standing there in front of him. And one of the three he bows to as The Lord. Then two of them go to Sodom and Gomorrah to save Lot and his family while Abraham stays back with The Lord, begging Him to not destroy all in those cities. So again, did Abraham actually see Christ, and even speak with Him in the Old Testament times? Yes.
What was the main difference between them and us then? It was that Christ was not born through woman yet to die on the cross, and seal The Gospel Salvation that was prophesied to come, even back in Genesis.
In the Book of Jude, a quote from the book of Enoch is given. Enoch, the seventh from Adam, prophesied of Christ's second coming...
Jude 14-15
14 And Enoch also, the seventh from Adam, prophesied of these, saying, Behold, the Lord cometh with ten thousands of His saints,
15 To execute judgment upon all, and to convince all that are ungodly among them of all their ungodly deeds which they have ungodly committed, and of all their hard speeches which ungodly sinners have spoken against Him.
KJV
Did Enoch understand about Christ's second coming? Of course he did. It was made known to him, even all the way back in those early days in Genesis. Would he also have known about The Gospel of Jesus Christ? Yes, which is why Enoch mentioned Jesus returning with ten thousands of His saints.
All that I could find is that Enoch walked with god, show the verse that speaks of a gospel being preached to him please.
Man's Bible chronology doesn't consider this, but tries to teach The Gospel of Jesus Christ wasn't understood by anyone back in OT times. That simply is not true. Isaiah was given to prophesy of Christ's crucifixion, so was David.
So the men you follow that have devised Hyper-Dispensationalism haven't studied enough of their Bible to even consider these things from Scripture. Yet they are there in God's Word. What those men have done instead is build themselves a business, using money from the gullible.