Graham Stanton has a very helpful little article on Paul's "gospel" in this book, edited by Dunn. He concludes that the gospel is "the glad tidings of God's once for all dynamic salvific act through Christ" (182). That is, "'Christ crucified for our salvation' is the central theme of Paul's gospel." (178).
I call his brief treatment helpful because it avoids some of the puzzling but fashionable expositions of the gospel on market today (N. T. Wright's and Michael Gorman's proposal, for example, that the gospel is "Jesus is Lord"). Stanton rightly fixes on 1 Cor 15:1-8 as a (the?) central text for explicating Paul's gospel, and also rightly makes Paul's gospel vertical first and horizontal derivatively. He also helpfully shows that while "gospel/good news" would be heard against the backdrop of the imperial cult (i.e. "Caesar is Lord"), Paul is mainly contrasting his use of gospel with the secular use, rather than employing it in the same way (i.e. referring mainly to lordship/authority).