"For I delivered unto you first of all, that which I also received, how that Christ died for our sins, according to the Scriptures, and that he was buried, and that he rose again the third day, according to the Scriptures."
That is the Gospel in a nutshell. Of course, there is a sense that the Gospel includes everything in the Bible, from Genesis 1:1 to Revelation 22:21. The whole Bible is about our Lord Jesus Christ, and the Gospel centers upon him. One of the 17th century Puritans put it this way:
"To accept Christ's righteousness, his blood alone for salvation, is the sum of the gospel." Thomas Wilcox (1621 - 1687)
Recommended Gospel sermons:
Compel Them to Come In, by Pastor Charles Haddon Spurgeon
When Government Tries To Be God, by Dr. David P. Murray
The Gospel According to the Risen Christ, by Rev. Cornelis Pronk
Recommended Gospel reading:
Faith Alone: The Evangelical Doctrine of Justification? by R. C. Sproul
The Romans Road to Salvation - my post