The gospel message is simpler than you make it.
Galatians 3:6-9 (NASB) 6 Even so Abraham BELIEVED GOD, AND IT WAS RECKONED TO HIM AS RIGHTEOUSNESS. 7 Therefore, be sure that it is those who are of faith who are sons of Abraham. 8 The Scripture, foreseeing that God would justify the Gentiles by faith,
preached the gospel beforehand to Abraham, saying,
"ALL THE NATIONS WILL BE BLESSED IN YOU." 9 So then those who are of faith are blessed with
Abraham, the believer.
According to Paul, the gospel message was preached to Abraham in one sentence.
But I'll answer your question.
Sometimes it's easier to see in the KJV:
Gen 15:5-6 KJV 5 And he brought him forth abroad, and said, Look now toward heaven, and tell the stars, if thou be able to number them: and he said unto him, So shall thy seed be. 6 And he believed in the LORD; and he counted it to him for righteousness.
You asked me to cite the gospel as it was given to Abram in Genesis. This verse is exactly that. We know this to be true because God makes Abraham righteous for his faith in it. When God says to Abraham "so shall thy seed be" He was talking about Jesus Christ - Paul tells us this:
Gal 3:16 NASB 16 Now the promises were spoken to Abraham and to his seed. He does not say, "And to seeds," as [referring] to many, but [rather] to one, "And to your seed," that is, Christ.
So when God says "so shall thy seed be" that seed he was talking about was Jesus Christ (according to Paul). It wasn't that Abraham believed any random thing that God told him and then God makes him righteous for it. God tells Abraham regarding the function and blessing of Christ and this is what "the gospel" essentially is.
I also posted Genesis 17;
Gen 17:7 KJV 7 And I will establish my covenant between me and thee and thy seed after thee in their generations for an everlasting covenant, to be a God unto thee, and to thy seed after thee.
Again, according to Paul in Gal 3:16, this "seed" that is being talked about is Christ. Now I posted the context of Genesis 17:7 to show you how God changes Abram's name to Abraham (signifying new birth and a new creation) but then showing how the promise that he would be heir of the world was to Abraham and his descendants or "seed." It said he would be the "Father of many nations" (not the father of the one nation of Israel), and then Genesis 17:7 says that those descendants (again from the many nations) would inherit this promise... thereby inheriting the righteousness God gave Abraham in Genesis 15:6 for his faith in the gospel of Jesus Christ. I also posted Isaiah 54:1 showing that the "adopted" offspring of Abraham (which is what God is talking about when He makes Abraham the Father of Many Nations) outnumber the physical descendants.
This is also why I posted Gal 3:6-9 and Romans 4:3-13. This demonstrates that those who have the same faith in the gospel of Jesus Christ that Abraham had are considered the adopted descendants of Abraham and thereby inherit Christ's righteousness which was given to Abraham in Genesis 15:5-6 for his faith in the gospel of Christ.
I posted John 8 to demonstrate that Jesus himself agrees with this interpretation as He condemns those Jews who don't have faith in him by stating their "father" is in fact not God and not Abraham, but their father was the devil... showing God views this kinship from Genesis as a spiritual kinship. Finally, Jesus confirms that Abraham did have faith in the gospel by stating "Your father Abraham rejoiced to see My day, and he saw it and was glad."
In Galatians, Paul talks about 2 covenants. The first is the covenant of FAITH IN THE GOSPEL, and the second is THE LAW OF MOSES. In Galatians 4, Paul compares these two covenants to the two wives of Abraham:
Gal 4:22-31 NASB 22 For it is written that Abraham had two sons, one by the bondwoman and one by the free woman. 23 But the son by the bondwoman was born according to the flesh, and the son by the free woman through the promise. 24 This is allegorically speaking, for these [women] are two covenants: one [proceeding] from Mount Sinai bearing children who are to be slaves; she is Hagar. 25 Now this Hagar is Mount Sinai in Arabia and corresponds to the present Jerusalem, for she is in slavery with her children. 26 But the Jerusalem above is free; she is our mother. 27 For it is written, "REJOICE, BARREN WOMAN WHO DOES NOT BEAR; BREAK FORTH AND SHOUT, YOU WHO ARE NOT IN LABOR; FOR MORE NUMEROUS ARE THE CHILDREN OF THE DESOLATE THAN OF THE ONE WHO HAS A HUSBAND." 28 And you brethren, like Isaac, are children of promise. 29 But as at that time he who was born according to the flesh persecuted him [who was born] according to the Spirit, so it is now also. 30 But what does the Scripture say? "CAST OUT THE BONDWOMAN AND HER SON, FOR THE SON OF THE BONDWOMAN SHALL NOT BE AN HEIR WITH THE SON OF THE FREE WOMAN." 31 So then, brethren, we are not children of a bondwoman, but of the free woman.
Paul had just gotten done saying that the true children of Abraham (and therefore, those who inherit Christ's righteousness) are the Adopted children of faith, not the physical descendants. He quotes Isaiah 54:1 describing it's meaning as showing that the adopted children outnumber the physical children.
Gal 3:6-8 NASB 6 Even so Abraham BELIEVED GOD, AND IT WAS RECKONED TO HIM AS RIGHTEOUSNESS. 7 Therefore, be sure that it is those who are of faith who are sons of Abraham. 8 The Scripture, foreseeing that God would justify the Gentiles by faith, preached the gospel beforehand to Abraham, [saying,] "ALL THE NATIONS WILL BE BLESSED IN YOU." 9 So then those who are of faith are blessed with Abraham, the believer.
26 For you are all sons of God through faith in Christ Jesus.
29 And if you belong to Christ, then you are Abraham's descendants, heirs according to promise.
Rom 4:9-13 NASB 9 Is this blessing then on the circumcised, or on the uncircumcised also? For we say, "FAITH WAS CREDITED TO ABRAHAM AS RIGHTEOUSNESS." 10 How then was it credited? While he was circumcised, or uncircumcised? Not while circumcised, but while uncircumcised; 11 and he received the sign of circumcision, a seal of the righteousness of the faith which he had while uncircumcised, so that he might be the father of all who believe without being circumcised, that righteousness might be credited to them, 12 and the father of circumcision to those who not only are of the circumcision, but who also follow in the steps of the faith of our father Abraham which he had while uncircumcised. 13 For the promise to Abraham or to his descendants that he would be heir of the world was not through the Law, but through the righteousness of faith.
Rom 8:15-17 NASB 15 For you have not received a spirit of slavery leading to fear again, but you have received a spirit of adoption as sons by which we cry out, "Abba! Father!" 16 The Spirit Himself testifies with our spirit that we are children of God, 17 and if children, heirs also, heirs of God and fellow heirs with Christ, if indeed we suffer with [Him] so that we may also be glorified with [Him.]
Paul makes the point in Galatians 3 that the covenant of faith in the gospel of Christ came 430 years BEFORE the covenant of The Law:
Gal 3:15-18 NASB 15 Brethren, I speak in terms of human relations: even though it is [only] a man's covenant, yet when it has been ratified, no one sets it aside or adds conditions to it. 16 Now the promises were spoken to Abraham and to his seed. He does not say, "And to seeds," as [referring] to many, but [rather] to one, "And to your seed," that is, Christ. 17 What I am saying is this: the Law, which came four hundred and thirty years later, does not invalidate a covenant previously ratified by God, so as to nullify the promise. 18 For if the inheritance is based on law, it is no longer based on a promise; but God has granted it to Abraham by means of a promise.
So the covenant of faith was ratified by God himself in the latter half of Genesis 15 when God has Abraham split some animals in half, and then God passes between the halves as a pilar of fire. This was 430 years before the law of Moses was given, and Paul makes that point that a covenant, once ratified, can't have conditions added to it.