What is the gospel?

I am amazed that you are so quickly turning away from the One who called you with Christ’s undeserved kindness to another sort of good news. Not that there is another good news; but there are certain ones who are causing you trouble and wanting to distort the good news about the Christ. However, even if we or an angel out of heaven were to declare to you as good news something beyond the good news we declared to you, let him be accursed. As we have said before, I now say again, Whoever is declaring to you as good news something beyond what you accepted, let him be accursed.

Now I remind you, brothers, of the good news that I declared to you, which you also accepted, and for which you have taken your stand. Through it you are also being saved if you hold firmly to the good news I declared to you, unless you became believers for nothing.
 For among the first things I handed on to you was what I also received, that Christ died for our sins according to the Scriptures; and that he was buried, yes, that he was raised up on the third day according to the Scriptures; and that he appeared to Ceʹphas, and then to the Twelve. After that he appeared to more than 500 brothers at one time, most of whom are still with us, though some have fallen asleep in death. After that he appeared to James, then to all the apostles. But last of all he appeared also to me as if to one born prematurely.
For I am the least of the apostles, and I am not worthy of being called an apostle, because I persecuted the congregation of God. But by God’s undeserved kindness I am what I am. And his undeserved kindness to me was not in vain, but I labored more than all of them; yet it was not I, but the undeserved kindness of God that is with me. Whether, then, it is I or they, this is the way we preach, and this is the way you believed.