Jesus was sent to invite his brethren into God's family. The gospel was sent to his brethren first because the Hebrews of that time comprised the firstborn of the Father. Some accepted Jesus' invitation and some didn't, just like in the world today.
After Jesus was executed his apostles carried on his work. Paul was a Roman citizen who received his early education in a large Gentile city and so he was intimately familiar with Gentile culture whereas the Twelve were only familiar with the culture of Judea.
Jesus sent Paul to invite Jews and Gentiles into the kingdom of God.
Acts 14:21-22 And when they had preached the gospel to that city (Derbe) and made many disciples, they returned to Lystra, Iconium, and Antioch strengthening the souls of the disciples, exhorting them to continue in the faith, and saying, “We must through many tribulations enter the kingdom of God.”
The Jews were familiar with Jesus of Nazareth and were aware of his execution because of having attended the Passover in Jerusalem. Gentiles for the most part did not know about Jesus' death, burial and resurrection so Paul had to cover some ground that Jews were well aware of.
1 Corinthians 15:1-4 is not some new gospel, rather it was a reminder to them that Paul had already covered that ground.
When all is done Jesus will deliver the kingdom to the Father, the Most High.
In 1 Corinthians 15:24 Paul states that in the end Jesus will deliver the kingdom to God the Father.
That is the one and only gospel, the good news of God's family.
