The Gospel, in-a-nutshell, is this: "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" (1 Corinthians 15:3-4 - KJV).
The Gospel was offered to the Jews first, and because the vast majority of the Jews rejected Christ, the Gospel was then offered to the non-Jews, that is, the Gentiles (See John 1:11-12)! Unfortunately, the vast majority of Gentiles have also rejected Jesus as well. Man prefers to trust in the "rules and regulations" of the multitude of religions of the world, believing they can earn the right to go to paradise. They believe that they can achieve the necessary right-standing (have their own righteousness) with God by religious works, and are not willing to face the fact that religious works can not make them perfect (totally righteous). Paul wrote of them saying: "For they being ignorant of God's righteousness, and going about to establish their own righteousness, have not submitted themselves unto the righteousness of God" (Romans 10:3 - KJV).
The first four books of the New Testament are known as the Gospels of Matthew, Mark, Luke, and John. Mark's Gospel even begins with these words: "The beginning of the Gospel of Jesus Christ, the Son of God" (Mark 1:1 - KJV)! These four writers share their knowledge of the life, the death, and the Resurrection of Jesus Christ. John concludes his eye-witness account by writing: "This is the disciple who testifies of these things, and wrote these things: and we know that his testimony is true. And there are also many other things which Jesus did, that should they all be included, I suppose that even the world itself could not contain the books that would need to be written. Amen" (John 21:34-25 - SLV). In John's first epistle (letter), he wrote: "These things have I written unto you that believe on the name of the Son of God; that you may know that you have eternal life..." (1 John 5:13 - KJV)!
So, there it is; God's offer of salvation to any and all! Trust in Jesus, the Son of God!
Whosoever shall call upon Jesus to be his Lord, shall be saved (Romans 10:9-10)!