Before reading this feel free to look up the word in a concordance and see how it is used. I Cor. 15:1-4 and Rom. 1:1-4 are probably the two clearest passages. The Gospel of Jesus Christ is the Good News of what He did for us. Our response to the Gospel is not the gospel. Think about it. Acts 2 after Peter tells them about who Jesus really is, what he really did and what their condition was, they ask what to do. Acts 2:37-39 is how to respond to the Gospel that had been preached. I can prove that the individual's response is not part of the Gospel.
Paul wrote that the gospel that he preached came directly from Jesus. To make sure that no one misunderstood, he also said that it did not come from man, not after man, nor was it taught by a man (Gal. 1:11-12). Acts 9:6 says that Paul was told by Jesus to go into the city where he would be told what to do. Acts 22:16 verifies that a man, Annanias, told him to be baptized. If baptism is a part of the gospel then he did learn part of it from, after, or taught by a man. Baptism was not in the message Jesus delivered in person to Saul of Tarsus on the road. All he learned there was the good news of a resurrected Jesus. I realize at the time it was a shock, but that does not make it bad news.
Gospel is a word that should be used only to describe good news. The gospel of Jesus Christ is the story of Jesus, what he did to save fallen men. Gospel is not a term that should be used to describe everything that is sound doctrine. In fact, it is not sound to call all truth the gospel.
The scriptures do teach that repentance and baptism are required if one is to receive forgiveness of sins and the gift of the Holy Spirit. That is sound doctrine. So is teaching against idolatry, adultery, theft, laziness, etc., but none of those are the Gospel.
15 And He said unto them, Go ye into all the world and preach the gospel to the whole creation. 16 He that believeth and is baptized shall be saved: but he that disbelieveth shall be condemned. Mark 16 ASV.
Obeying the Gospel (Rom. 6) is not doing the orders of the gospel. It is becoming conformed to His death, BURIAL, and resurrection. The apostles had the information about Jesus before Pentecost, but the "how to be saved" part came by inspiration of the Holy Spirit later on Pentecost.
The "gospel of Christ" is the most important phrase we can use correctly. - ELIHU