In 1 Corinthians 15:1-4, Paul clearly states: 1 Moreover, brethren, I declare to you the gospel which I preached to you, which also you received and in which you stand, 2 by which also you are saved, if you hold fast that word which I preached to you--unless you believed in vain. 3 For I delivered to you first of all that which I also received: that Christ died for our sins according to the Scriptures, 4 and that He was buried, and that He rose again the third day according to the Scriptures.
In Ephesians 3:1-9, Paul clearly states: Ephesians 3:1 - For this reason I, Paul, the prisoner of Christ Jesus for you Gentiles— 2 if indeed you have heard of the dispensation of the grace of God which was given to me for you, 3 how that by revelation He made known to me the mystery (as I have briefly written already, 4 by which, when you read, you may understand my knowledge in the mystery of Christ), 5 which in other ages was not made known to the sons of men, as it has now been revealed by the Spirit to His holy apostles and prophets: 6 that the Gentiles should be fellow heirs, of the same body, and partakers of His promise in Christ through the gospel, 7 of which I became a minister according to the gift of the grace of God given to me by the effective working of His power. 8 To me, who am less than the least of all the saints, this grace was given, that I should preach among the Gentiles the unsearchable riches of Christ, 9 and to make all see what is the fellowship of the mystery, which from the beginning of the ages has been hidden in God who created all things through Jesus Christ.
In Galatians 1:6-9, we see that false teachers were troubling the Galatians and were perverting the gospel that Paul received from Christ by revelation. The Judaizers' perverted the gospel by "adding" requirements (laws) from the Old Covenant as necessary prerequisites to salvation and we have misguided, modern day Judaizers' today who pervert the gospel by "adding" requirements from the OT law, which results in salvation by "grace plus law, faith plus works." Paul refers to this as a "different" gospel, which is no good news at all!
To BELIEVE the gospel is to trust in the death, burial and resurrection of Christ as the ALL-sufficient means of our salvation, yet sadly, the gospel continues to be perverted today by works-salvationists/modern day Judaizers/misquided teachers of the law.