What Is the Gospel, Biblically Defined?

For I am not ashamed of the gospel, for it is the power of God for salvation to everyone who believes.  (Romans 1:16)

For God so loved the world that He gave his only Son, that whoever believes in Him should not perish but have eternal life.  (John 3:16)

Now I would remind you, brothers, of the gospel I preached to you, which you received, in which you stand, and by which you are being saved, if you hold fast to the word I preached to you―unless you believed in vain. For I delivered to you as of first importance what I also received: that Christ died for our sins in accordance with the Scriptures, that He was buried, that He was raised on the third day in accordance with the Scriptures, and that He appeared to Cephas, then to the twelve. Then He appeared to more than five hundred brothers at one time, most of whom are still alive, though some have fallen asleep. Then He appeared to James, then to all the apostles. Last of all, as one untimely born, He appeared also to me.  (1 Corinthians 15:1-8)