I answered within your post I copied and pasted changing my reply to blue for clarification on who was saying what
And Peter was preaching a GOSPEL , called WHAT ?? Acts 2:38 Peter said to them, "Repent, and each of you be baptized in the name of Jesus Christ for the forgiveness of your sins; and you will receive the gift of the Holy Spirit. To be BAPTIZED for the FORGIVENESS of SINS and what SINS did they commit ?? Sin is defined in God's word as a transgression of the law so the answer would clearly be a transgression of the law correct isn't it elementary?
And what gospel did Paul preach , Eph 3:1-9 1 For this reason I, Paul, the prisoner of Christ Jesus for the sake of you Gentiles— 2 if indeed you have heard of the stewardship of God's grace which was given to me for you; 3 that by revelation there was made known to me the mystery, as I wrote before in brief. 4 By referring to this, when you read you can understand my insight into the mystery of Christ, 5 which in other generations was not made known to the sons of men, as it has now been revealed to His holy apostles and prophets in the Spirit; 6 to be specific, that the Gentiles are fellow heirs and fellow members of the body, and fellow partakers of the promise in Christ Jesus through the gospel, 7 of which I was made a minister, according to the gift of God's grace which was given to me according to the working of His power. 8 To me, the very least of all saints, this grace was given, to preach to the Gentiles the unfathomable riches of Christ, 9 and to bring to light what is the administration of the mystery which for ages has been hidden in God who created all things; and preached the MYSTERY , Rom 16:25
Now to Him who is able to establish you according to my gospel and the preaching of Jesus Christ, according to the revelation of the mystery which has been kept secret for long ages past,and 27 tCol 1:26 26 that is, the mystery which has been hidden from the past ages and generations, but has now been manifested to His saints, 27 to whom God willed to make known what is the riches of the glory of this mystery among the Gentiles, which is Christ in you, the hope of glory. 28 We proclaim Him, admonishing every man and teaching every man with all wisdom, so that we may present every man complete in Christ. 29 For this purpose also I labor, striving according to His power, which mightily works within me. a MYSTERY that was kept SECRET .
Notice I added the verses you reference so we can read them and in the Colossians verse added more context so we can see the full meaning.
The mystery gospel as you try to call it is the same gospel of Christ thar Peter taught it was a mystery in the fact that it was not reveiled to us gentiles the same time it was givin the Jews but now we are called by the same gospel both Jew and gentilr and that isn made clear in Acts 10 Please read Acts 10 so I don't have go copy and paste
And you mean that Paul and Peter preached the same GOSPEL ? Yes a hundred times yes the word of God clearly teaches this