Here is Dr. David Jeremiah's study note on the gospel from 1 Corinthians 15, in The Jeremiah Study Bible:
"[1 Corinthians] 15:3-7 Here is the standard by which every definition of the gospel must be measured. It must include four elements: Christ's death, burial, and resurrection as well as the testimony of the witnesses to the Resurrection. The gospel cannot be preached the way it should be without proclaiming all four truths."[1]
The reason I like Dr. Jeremiah's explanation of the gospel is that he makes it clear that the definition of the gospel includes not merely the two facts of Christ's death and resurrection, but rather four facts: "Christ died for our sins...was buried...was raised...and was seen" (1 Cor. 15:3-5). Not a popular truth for some people today, but it's biblical!
ENDNOTE:
[1] Dr. David Jeremiah, The Jeremiah Study Bible, note on 1 Corinthians 15:3-7.