Jesus’ Gospel was the gospel of the kingdom.
23 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. 24 And his fame went throughout all Syria: and they brought unto him all sick people that were taken with divers diseases and torments, and those which were possessed with devils, and those which were lunatick, and those that had the palsy; and he healed them. 25 And there followed him great multitudes of people from Galilee, and from Decapolis, and from Jerusalem, and from Judaea, and from beyond Jordan.Matthew 4:23–25
All of the apostles, including Paul, preached this gospel. Oftentimes, people quote this passage to define the gospel.
Moreover, brethren, I declare unto you the gospel which I preached unto you, which also ye have received, and wherein ye stand; 2 By which also ye are saved, if ye keep in memory what I preached unto you, unless ye have believed in vain. 3 For I delivered unto you first of all that which I also received, how 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:1 Corinthians 15:1–4
This is part of the gospel, but it is not the whole of the gospel. Why did Jesus die, become buried, and rise from the dead? So that we could be part of His Kingdom!