Your argument is about two gospels. One for Israel and another (through Paul) to the Gentiles. Doesn't matter whether you're saying a previous gospel for Israel is now dead, that's still preaching the idea of two gospels.
But you are even wrong about the gospel to Israel being now dead...showing you have not understood what Promise it was that Abraham believed from the start...
Rom 4:8-16
8 Blessed is the man to whom the Lord will not impute sin.
9 Cometh this blessedness then upon the circumcision only, or upon the uncircumcision also? for we say that faith was reckoned to Abraham for righteousness.
In Galatians, Apostle Paul says those of Faith are the children of Abraham. Those in Christ Jesus have believed the same Faith as Abraham did.
10 How was it then reckoned? when he was in circumcision, or in uncircumcision? Not in circumcision, but in uncircumcision.
When did Abraham believe? Before God gave him the covenant of flesh circumcision, or after? The Promise by Faith through Abraham was reckoned FIRST, before the covenant of flesh circumcision.
11 And he received the sign of circumcision, a seal of the righteousness of the faith which he had yet being uncircumcised: that he might be the father of all them that believe, though they be not circumcised; that righteousness might be imputed unto them also:
Abraham received the sign of circumcision ONLY AFTER he had first believed the Promise by Faith. And this so he would be the father of all those who believe. That means all those who have believed on The Saviour Jesus Christ, i.e., Christians.
Does that sound like a different Gospel than what Christians have believed today? It isn't different, as Apostle Paul is showing here.
12 And the father of circumcision to them who are not of the circumcision only, but who also walk in the steps of that faith of our father Abraham, which he had being yet uncircumcised.
Is Abraham spiritual father to Israel also, even though they may be circumcised too? Yes when they believe by Faith like Abraham did, and not by the law. This is The Gospel of Jesus Christ, even though Paul's talking about an OT event with Abraham's Faith.
13 For the promise, that he should be the heir of the world, was not to Abraham, or to his seed, through the law, but through the righteousness of faith.
14 For if they which are of the law be heirs, faith is made void, and the promise made of none effect:
15 Because the law worketh wrath: for where no law is, there is no transgression.
16 Therefore it is of faith, that it might be by grace; to the end the promise might be sure to all the seed; not to that only which is of the law, but to that also which is of the faith of Abraham; who is the father of us all,
(KJV)
So what's Apostle Paul doing saying things to Gentiles like Abraham being the father (spiritual) of us all, if that's not the same Gospel?
Gal 3:13-14
13 Christ hath redeemed us from the curse of the law, being made a curse for us: for it is written, Cursed is every one that hangeth on a tree:
14 That the blessing of Abraham might come on the Gentiles through Jesus Christ; that we might receive the promise of the Spirit through faith.
(KJV)
There it is... Paul proclaiming that the blessing of Abraham involves the very SAME Gosepl that Paul preached to the Gentiles!
Here's Christ's declaration of that Promise given in Abraham's day...
John 8:56-58
56 Your father Abraham rejoiced to see My day: and he saw it, and was glad.
57 Then said the Jews unto Him, "Thou art not yet fifty years old, and hast Thou seen Abraham?"
58 Jesus said unto them, "Verily, verily, I say unto you, Before Abraham was, I am."
(KJV)
