I see nothing about where the disciples were preaching the death, burial, and resurrection of Jesus Christ for sins. That is the gospel Paul preached.
See Acts18 re: Paul in Corinth. Note Acts18:11 re: Paul's time in Corinth. Note Acts18:5 for the Spirit compelled focus - Jesus is the Christ.
See Acts 13:16-41 for the Gospel Paul was proclaiming to both Jews and Greeks. Note within these verses the death, burial, resurrection (which Paul reminds the Corinthians of in 1Cor15). Note Acts13:33 where Paul references Psalm2. Read Psalm2 and note how it explains what "Christ" (Anointed) means and refers to; YHWH's Anointed/Christ - YHWH's KING - to whom He will give the earth - to whom all kings are subservient and whom they best kiss lest He be angry.
See 1Cor3:10-11 re: the foundation Paul laid - Jesus is the Christ (all explained above).
1Cor15 is not "the Gospel" but a summary of points of the Gospel Paul had taught the Corinthians in order to explain to them the necessity of belief in Jesus' resurrection which was proof of the foundation Paul had previous laid - Jesus is the Christ, YHWH's Anointed, YHWH's KING who inherited the earth and to whom all men are subservient.
We should be evangelizing with Psalm2 just as Paul did. Then more would understand who and what Jesus is - what "Christ" means.
The Kingdom carries over from pre-cross to post resurrection. The King was on earth. The resurrected King is at the right hand of the Father with all authority in Heaven and on earth. It's always been and still is about the King and the Kingdom.