What is the Gospel?
In short, it is stated in 1 Corinthians 15:1-4 which says
1 Moreover, brethren, I declare to you the gospel which I preached to you, which also you received and in which you stand, 2 by which also you are saved, if you hold fast that word which I preached to you — unless you believed in vain. 3 For I delivered to you first of all that which I also received: that Christ died for our sins according to the Scriptures, 4 and that He was buried, and that He rose again the third day according to the Scriptures. (NKJV)
For more detail and biblical context please see:
What does it mean to be saved?
What is the Gospel?
What's the Gospel?
From Presentation Library
