The Gospel is the good news that we have forgiveness of sins through Jesus. Specifically, the Gospel is defined by Paul in 1 Cor. 15:1-4, "Now I make known to you, brethren, the gospel which I preached to you, which also you received, in which also you stand, by which also you are saved, if you hold fast the word which 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 according to the Scriptures, and that He was buried, and that He was raised on the third day according to the Scriptures."
The Gospel comes from God (Gal. 1:10-12), is the power of God for salvation (Rom. 1:16), is a mystery (Eph. 6:19), and is a source of hope (Col. 1:23), faith (Acts 15:9), life (2 Cor. 4:14), and peace (Eph. 6:15).
So, salvation is found in the Gospel. In other words, when a person trusts in what Christ did on earth (lived a perfect life), bore our sins in His body on the cross (1 Pet. 2:24), and died with them, paying the price for our redemption (Acts 20:28, Col. 2:14) according to the Law, then that person is saved from God's righteous judgment. We are declared right in God's eyes through Jesus' sacrifice on the cross.
The four Gospels are known as Matthew, Mark, Luke, and John. Matthew, Mark, and Luke are known as the synoptic Gospels.