The gospel of our salvation is in 1cor15:1-4:
(I Corinthians 15:1-4 [KJV])  Moreover, brethren, I declare unto you the gospel which I preached unto you, which also ye have received, and wherein ye stand;  By which also ye are saved, if ye keep in memory what I preached unto you, unless ye have believed in vain.  For I delivered unto you first of all that which I also received, how that Christ died for our sins according to the scriptures;  And that he was buried, and that he rose again the third day according to the scriptures:
(Matthew 4:23 [KJV]) And Jesus went about all Galilee, teaching in their synagogues, and preaching the gospel of the kingdom, and healing all manner of sickness and all manner of disease among the people.
So we can see from this that the gospel was already preached before Christ resurrection, before Paul. Christ preached the gospel already in Matthew.
Enter into life
The gospel is our entrance into salvation as stated in Ephesians 3:6
(Ephesians 3:6 [KJV]) That the Gentiles should be fellowheirs, and of the same body, and partakers of his promise in Christ by the gospel:
So we see that the Gentiles should be fellowheirs by the gospel and of the same body as the jews by the gospel and partakers of his promise in Christ by the gospel.
Judged by the gospel
And it is by Pauls gospel that we shall be eternally judged.
In the day when God shall judge the secrets of men by Jesus Christ according to my gospel. — KJV, Romans 2:16
Believed in Jesus Christ
Galatians 2:16 KJV  Knowing that a man is not justified by the works of the law, but by the faith of Jesus Christ, even we have believed in Jesus Christ, that we might be justified by the faith of Christ, and not by the works of the law: for by the works of the law shall no flesh be justified.
It’s by grace through faith in Christ alone that I have been eternally saved.