You mean there is only one Gospel TODAY, or only one Gospel in the entire N.T.?
I assume you believe the latter, so what is "the only one" Gospel called?
Until we get nailed down exactly what each of us are talking about, communication is impossible.
I believe that Jesus preached the gospel, and Paul preached the same gospel. Only he preached it to the gentiles who didn't know the laws.
All the apostles preached the same gospel and all preached salvation through faith by the grace of God. They all preached about faith and grace.
1 Peter 1
Who are kept by the power of God through faith unto salvation ready to be revealed in the last time.
Jude
But ye, beloved, building up yourselves on your most holy faith, praying in the Holy Ghost, Keep yourselves in the love of God, looking for the mercy of our Lord Jesus Christ unto eternal life.
Jesus in Luke 7
And he said to the woman, Thy faith hath saved thee; go in peace.
Paul in Ephesians 2
For by grace are ye saved through faith; and that not of yourselves: it is the gift of God:
John in 1John :1
For whatsoever is born of God overcometh the world: and this is the victory that overcometh the world, even our faith.
John in John 1
And of his fulness have all we received, and grace for grace. For the law was given by Moses, but grace and truth came by Jesus Christ.
And in 2 John 1
Grace be with you, mercy, and peace, from God the Father, and from the Lord Jesus Christ, the Son of the Father, in truth and love.
Peter in 2 peter
But grow in grace, and in the knowledge of our Lord and Saviour Jesus Christ. To him be glory both now and for ever. Amen
1 Peter 5
But the God of all grace, who hath called us unto his eternal glory by Christ Jesus, after that ye have suffered a while, make you perfect, stablish, strengthen, settle you
Jude
For there are certain men crept in unawares, who were before of old ordained to this condemnation, ungodly men, turning the
grace of our Godinto lasciviousness, and denying the only Lord God, and our Lord Jesus Christ
There is only one gospel.