I have shown you, and Peter did not change it after Paul's conversion, for as said, Paul's gospel was that of "testifying both to the Jews, and also to the Greeks, repentance toward God, and faith toward our Lord Jesus Christ. (Acts 20:21; cf. 26:20)
And before Paul even mentioned the Lord Jesus to the barbarians in Acts 14, he even told them,
"We also are men of like passions with you, and preach unto you that ye should turn from these vanities unto the living God, which made heaven, and earth, and the sea, and all things that are therein:" (Acts 14:15)
Thus according to HD understanding, Paul must be preaching salvation by works, rather than being consistent with Christ, Peter and himself in calling souls to salvation by faith, but a faith which changes lives by God's grace.
And thus attempting to make Paul's gospel different based on his use of "my gospel" will not work, for it is not shown to be salvifically different that Peter's or Johns, etc.
For Paul states that while he received the truth of the gospel directly from God, he "delivered unto you first of all that which I also received, how that Christ died for our sins according to the scriptures...; (1 Corinthians 15:3)
Remember that Jesus Christ of the seed of David was raised from the dead according to my gospel : (2 Timothy 2:8)
This, and that, as Peter whom you think preached a different gospel says, "ye were not redeemed with corruptible things, as silver and gold, from your vain conversation received by tradition from your fathers; But with the precious blood of Christ, as of a lamb without blemish and without spot:" (1 Peter 1:18-19)
And of repentance toward God, and faith toward our Lord Jesus Christ, (Acts 20:21) whom Peter and Paul both affirm is Lord, and by a manner of faith that confesses Christ, (Rm. 10:10) and effects works by love, salvifically constituted "the gospel of the grace of God," (Acts 20:24) and which both Peter and Paul preached.
And as said, if Peter preached a different gospel than Paul's - "what we have preached unto you" then he was accursed. (Gal. 1:6-9) And in which book he goes on to teach that "we" means one in which the other apostles concurred with, and which did not teach salvation by the works of the law, even though faith effects fruit the obedience of faith.
As for "the revelation of the mystery, which was kept secret since the world began, But now is made manifest, and by the scriptures of the prophets, according to the commandment of the everlasting God, made known to all nations for the obedience of faith: (Romans 16:25,26) the gospel is first a mystery because what both Peter and Paul preached was hide it from men, which is why Is. 6:9,10 is the or about the most referenced OT text in the gospels and NT (Mt 13:14,15; Mr 4:12; Lu 8:10; Joh 12:40 Ac 28:26,27; Ro 11:8)
And he said, Go, and tell this people, Hear ye indeed, but understand not; and see ye indeed, but perceive not. Make the heart of this people fat, and make their ears heavy, and shut their eyes; lest they see with their eyes, and hear with their ears, and understand with their heart, and convert, and be healed. (Isaiah 6:9-10)
Which none of the princes of this world knew: for had they known it, they would not have crucified the Lord of glory. (1 Corinthians 2:8)
Secondly "the mystery of the gospel" most fully includes that which pertains to their being one gospel, not two:
Whereby, when ye read, ye may understand my knowledge in the mystery of Christ) Which in other ages was not made known unto the sons of men, as it is now revealed unto his holy apostles and prophets by the Spirit; That the Gentiles should be fellowheirs, and of the same body, and partakers of his promise in Christ by the gospel: (Ephesians 3:4-6)
Although salvation by grace was taught by the Lord Himself, as seen in John, (Jn. 3:16; 5:24; 10:27-29; 11:25,26) and typified in healings in the synoptics, and by Peter afterward as explained, this aspect of the Jews and Gentiles being of the same body was a mystery, even though the Lord alluded to it in John.
And which was revealed to Paul, and also the theology behind salvation by grace, under which the penitent publican, and so-called "good thief" and Mary and Martha, etc. were also saved by. Thanks be to God thru Jesus the Lord.
I don’t think anyone said there was any “salvifically different” teaching other than the focus, emphasis and requirements during each dispensation. The dispensation changed at Pentecost. As smvoice showed in her post about 1 Cor. 3:10,11 there is the “according to the grace” and I pointed out that it was different than the “according to the law”.
Each of the Gospels were preached to a different group of people .. so I think it might account for some of the differences.
For example, Matthew preached to the Jews (and he made a lot of references to the Old Testament) with which the Jews would have been very familiar.
Each of the four Gospels had a different focus.