There are at least two definitions of "the Gospel" in the New Testament. The first is given in 1 Corinthians 15:
HCSB 1 Cor 15:1 Now brothers, I want to clarify for you the gospel I proclaimed to you; you received it and have taken your stand on it. 2 You are also saved by it, if you hold to the message I proclaimed to you—unless you believed to no purpose. 3 For I passed on to you as most important what I also received:So, according to this first definition, the Gospel is the death, burial, resurrection, and appearance of Christ.
that Christ died for our sins according to the Scriptures,
4 that He was buried,
that He was raised on the third day according to the Scriptures,
5 and that He appeared to Cephas, then to the Twelve.
6 Then He appeared to over 500 brothers at one time,
most of whom remain to the present, but some have fallen asleep.
7 Then He appeared to James, then to all the apostles.
8 Last of all, as to one abnormally born, He also appeared to me.
However, according to at least three other sources, the Gospel is that all nations will be blessed.
Highlights of Romans 10:14 - 12:1
Not all have obeyed the gospel; didn't they hear? Yes. Didn't Israel understand when Moses announced that they would be made jealous by the salvation of non-Jews, and when Isaiah said that God would be found by those not looking for Him, while the Jews, to whom God was spreading out His hands, remained disobedient and defiant? Yet God's mercy extends to both Jew and Gentile, to the Gentile who has been grafted into God's olive tree by faith, and to the Jew who can be re-grafted back into the tree if he'll stop abiding in unbelief. Regarding the gospel, the falling away of the Jews results in the salvation of Gentiles. Both groups have disobeyed, and God has imprisoned all in disobedience, so that He may have mercy on all. Therefore, worship God by presenting your bodies as a living sacrifice.
So we see in this summary that the gospel is that not just the Jews, who were chosen, but also the Gentiles, will be blessed with God's mercy.
Highlights of Galatians 2:11-16
Paul had to correct Peter publicly, because he was deviating from the truth of the gospel. Peter should have remembered that just as the Jews had been justified by faith, so too the Gentiles, so he should quit acting like the Gentiles were not part of the family.
So we see in this summary that deviating from the gospel is to forget that not just the Jews, who were chosen, but also the Gentiles, were blessed with God's mercy.
Highlights of Galatians 3:8
The Scripture foresaw that God would justify the Gentiles by faith and foretold this gospel to Abraham, saying, "All the nations will be blessed in you."
So we see in this summary that the gospel is that not just the Jews, who were chosen, but also the Gentiles, will be blessed.
So, in summation, there are at least two Biblical definitions in the New Testament for the term "Gospel" (or "Good News"):
1) the death, burial, resurrection, and appearance of Christ
2) that not only the chosen Jews, but also the Gentiles, would be blessed with salvation by faith in Christ.