The gospel of the kingdom is centered on the King, the Christ. The first message which Paul preached after being converted was the "gospel of the kingdom" in the synagogues of the Jews:
"And straightway he preached Christ in the synagogues, that he is the Son of God...proving that this is the very Christ" (Acts 9:20,22).
That was the same gospel which Paul continued to preach to the Jews:
"And Paul, as his manner was, went in unto them, and three sabbath days reasoned with them out of the scriptures, Opening and alleging, that Christ must needs have suffered, and risen again from the dead; and that this Jesus, whom I preach unto you, is Christ" (Acts17:2,30).
That is the same message that Apollos preached to the Jews:
"For he mightily convinced the Jews, and that publicly, shewing by the scriptures that Jesus is Christ" (Acts18:28).
That is the same "good news" which Peter preached to the Jews on the day of Pentecost:
"Therefore let all the house of Israel know assuredly, that God hath made the same Jesus, whom ye have crucified, both Lord and Christ" (Acts 2:36).
So we can clearly see that the central message that was preached to the Jews is the fact that it is Jesus Who is the Christ, the Son of God. and those who believed that "good news" (gospel) were born of God and received eternal life.
Despite these facts you say that Peter and Paul and Apollos were preaching to the Jews that the Lord Jesus died for our sins.