The Greek word εὐαγγέλιον or another form of it is found 76 times in the New Testament. In the NAS, it is translated "gospel" 73 times, "gospel's" 2 times, and "good news" 1 time. Its literal meaning is "good tidings."
In the synoptic Gospels and John, the good news is that the divine-human Jewish Messiah had come to set up his eternal, spiritual kingdom, bringing salvation to all who would repent of their sins and put their faith in him. In the Christ, there was authority over sin, death, and the devil. Christ brought freedom! The climax of the Gospels is the recounting of Jesus' death and resurrection, which is at the core of the message of the gospel, and was anticipated in the Gospels, at least by Jesus himself, who knew that redemption and freedom could only come through giving up his life. The good news doesn't stop with salvation from sin; Jesus will resurrect those he has redeemed, and they will live and reign with him forever, while the wicked will be judged and condemned.
I Corinthians 15:1-4 declares that the gospel is that Christ died for our sins, was buried, and rose again, after which he showed himself alive to many people. (I Cor. 15 is the only place I've seen where the gospel is actually defined in Scripture and not just described.) It is only through the death and bodily resurrection of Christ that we can have forgiveness and cleansing of sin, become adopted into the family of God to be inhabited by the Spirit of God, enabled to live a holy life, doing that which is well-pleasing to God. The gospel message also includes the hope of a new body resurrected to live eternally with Christ in a new heaven and a new earth.
Consistent with the Gospels, the Epistles speak of the gospel as the truth -- something that should be defended and confirmed. Paul spoke of his responsibility to preach the gospel. He said that he was an ambassador of the mystery of the gospel, which he also calls the message of reconciliation, something all of us should proclaim as Christ's ambassadors:
"[God] through Christ reconciled us to himself and gave us the ministry of reconciliation; that is, in Christ God was reconciling the world to himself, not counting their trespasses against them, and entrusting to us the message of reconciliation. Therefore, we are ambassadors for Christ, God making his appeal through us. We implore you on behalf of Christ, be reconciled to God" (II Cor. 5:18-20).
The four aspects of the Gospel message are its benefits (salvation, which includes justification, regeneration, adoption, entire sanctification, and glorification), the Person bringing the benefits (the divine-human Jesus), the Work done by the Person to bring the benefits (the death and resurrection of Christ), and the way in which these benefits are appropriated (through faith). So the "good news" includes the Blessings, the Person, the Work, and the grace-enabled Response.
The blessing of salvation is more than just forgiveness. It also includes the privilege of being born again and sanctified into the likeness of Christ. Ultimately salvation includes the work of glorification.
The focus of the gospel message is on the Person, the one who brings the blessing by his Work. One could say that the Gospel is Christ himself. We must not forget that the only one who could bring salvation had to be both God and man as a single person to provide for us an adequate mediator who could represent both parties estranged from each other (God and man). At the center, the gospel is the work that this perfect God-man did (in time and space) by dying and rising from the dead so that we could be forgiven. The gospel also entails the truth of how salvation is appropriated—it is only through faith in Christ that we can be saved.
In Galatians 2, Paul addressed the danger of "another gospel" than the one that he had presented. This different Gospel was that salvation could be attained by the keeping of the Mosaic Law. Paul rejected that idea as a false gospel. It is only through faith in Christ that we can be saved. It makes sense that how one appropriates salvation is part of the gospel message because if salvation is through Christ (because of the divine-human Messiah's death and resurrection), then the only way it could be appropriated is simply through identifying with Christ in his death and resurrection. Adding the works of the law as a means of salvation would be a completely different way of salvation and would mean that Christ's work is not sufficient, and even in vain.
To summarize the gospel message, here's an attempt at a succinct definition: The gospel of the kingdom is the good news that we can be saved by faith in the crucified and risen divine-human Christ.
Or: The gospel is the good news that the divine-human Christ died and rose again to bring salvation to all who put their trust in him.
 Including justification, regeneration, adoption, entire sanctification, and future glorification.
 Mark 1:15 - “The time is fulfilled, and the kingdom of God is at hand. Repent, and believe in the gospel.” John 3:16 – For God so loved the world, that he gave his only begotten Son, that whoever believed in him would not perish but have everlasting life.
 Mark 2, John 11, I John 3:8.
 John 8:36 – "Therefore if the Son makes you free, you shall be free indeed."
 But Jesus answered them, saying, “The hour has come that the Son of Man should be glorified. Most assuredly, I say to you, unless a grain of wheat falls into the ground and dies, it remains alone; but if it dies, it produces much grain."
 Hebrews 9:22 – "And according to the law almost all things are purified with blood, and without shedding of blood there is no remission."
 Galatians 4:4-6 – "But when the fullness of the time had come, God sent forth His Son, born of a woman, born under the law, to redeem those who were under the law, that we might receive the adoption as sons. And because you are sons, God has sent forth the Spirit of His Son into your hearts, crying out, 'Abba, Father!'"
 Hebrews 13:12 – "Therefore Jesus also, that He might sanctify the people with His own blood, suffered outside the gate."
 Hebrews 13:20-21 – "Now may the God of peace who brought up our Lord Jesus from the dead, that great Shepherd of the sheep, through the blood of the everlasting covenant, make you complete in every good work to do His will, working in you what is well pleasing in His sight, through Jesus Christ, to whom be glory forever and ever. Amen."
 Philippians 3:20-21 – "For our citizenship is in heaven, from which we also eagerly wait for the Savior, the Lord Jesus Christ, who will transform our lowly body that it may be conformed to His glorious body, according to the working by which He is able even to subdue all things to Himself."
 Galatians 2:5; Ephesians 1:13; Philippians 1:7.
 Eph 6:19-20.
 Galatians uses the term 'blessing' to refer to what was appropriated by faith – "that the blessing of Abraham might come upon the Gentiles in Christ Jesus, that we might receive the promise of the Spirit through faith" (Galatians 3:14).
 Galatians 2:21 -- "I do not nullify the grace of God, for if righteousness were through the law, then Christ died for no purpose."