The Gospel, stated in its simplest form in 1 Corinthians 15:1-4, says that Jesus died on a cross according to the Scriptures, was buried, and rose again according to the Scriptures. The Gospel is the good news that God recognized mankind's need for a Savior, and as planned before the foundation of the world, Jesus offered Himself to be our Savior (Eph. 1:4; 1 Pet. 1:19-20; Rev. 13:8).
So, if there is only one Gospel, what about Mark 1:1 which declares that Mark's Gospel, in its entirety, is the Gospel? Or what about Matthew and Mark writing of the Gospel of the Kingdom? And, what about Paul having a Gospel to the Gentiles and Peter having a Gospel to the Jews?
It is quite simple. Mark's Gospel is a witness to the death, burial, and resurrection of Jesus; it is the Gospel "fleshed-out" in the life of our Savior. The person Matthew and Mark present, is in fact, the King of the Jew's long-awaited kingdom; He is their Messiah, their Anointed One, their Christ. And as for Paul and Peter, they preached the same message to those to whom they were sent. There is only one Gospel. It involves God's love, man's need, and the Solution, Who is both the Lamb, and the soon-coming Lion (1 Cor. 5:7; Rev. 5:5).
The one Gospel is called:
***The "Gospel of God"" (Rom. 1:1; 15:16).
***The "Gospel of His Son (Rom. 1:9).
***The "Gospel of Christ" (Rom. 1:16; 15:19, 29).
***The "Gospel of Peace" (Rom. 10:15).
***The "Gospel" (Rom. 1:15; 10:16; 11:28; 15:20).
***Paul called it "My Gospel" (Rom. 2:16; 16:25).
***The "Gospel of the Kingdom" (Matt. 4:23; 9:35; 24:14; Mk. 1:14).
If you fail to accept salvation as a gift by adding anything to it, what you are trusting in is not the Gospel, but a lie (Gal. 1:6-11). "Jesus answered and said unto them, This is the work of God, that ye believe on Him whom He hath sent" (Jn. 6:29). "Neither is there salvation in any other: for there is none other name under heaven given among men, whereby we must be saved" (Acts 4:12). "Jesus saith unto him, I am The Way, The Truth, and The Life: no man cometh unto the Father, but by Me" (Jn. 14:6). JESUS IS THE GOSPEL.