Salvation is right: the whole Bible is the gospel message.
The simple fact is that gospel can be used to refer to different things.
As Protestant J. Hampton Keathley III delineates:
(1) The gospel of Jesus Christ (Mark 1:1; 1 Cor. 9:12) and the gospel of His Son (Rom. 1:9). These two descriptions speak of the good news of salvation that comes through the person and work of Jesus Christ who is the very Son of God in human flesh. Again, this is a good news of deliverance from sin’s penalty, power and presence through the two advents of Christ.
(2) The gospel of the grace of God (Acts 20:24) emphasizes that salvation in all of its aspects is on the basis of grace rather than on some meritorious system of works.
(3) The gospel of the kingdom (Matt. 4:23; 9:35; 24:14) is the good news that God will establish His kingdom on earth through the two advents of the Lord Jesus Christ.
(4) The gospel of peace (Eph. 6:15) describes how this good news of salvation in Christ brings peace in all its many aspects (peace with God, the peace of God, peace with others, and world peace) through the victory accomplished by the Savior.
(5) The eternal or everlasting gospel (Rev. 14:6) expands our perspective of gospel as we normally think of it. This gospel as proclaimed by the angel has several key elements of gloriously good news that are developed in three commands and two reasons...
The same Protestant author goes on to point out:
Popular Notions
Limit the Meaning of the Gospel
Popular notions about the term ‘gospel’ tend to limit it to the message of how one may receive eternal life through faith in Christ, but it is much broader than that. For instance, Paul says in Romans 1:16-17, “For I am not ashamed of the gospel, for it is God’s power for salvation to everyone who believes, to the Jew first and also to the Greek. For the righteousness of God is revealed in the gospel from faith to faith, just as it is written, “the righteous by faith will live.” But by using the term “gospel” here, Paul is not...
His conclusion?
“Conclusion
“In summary, what is the gospel? It is the message of the good news of salvation, the word of truth offered to mankind by grace through faith in the finished work of Christ on the cross. It is a message not only of eternal life, but one that encompasses the total plan of God to redeem people from the ravages of sin, death, Satan, and the curse that now covers the earth.”
So when Paul says “But though we, or an angel from heaven, preach any other gospel unto you than that which we have preached unto you, let him be accursed” we can consider the Catholic Church accursed because the apostles didn’t teach about the assumption of Mary!
What was the gospel that Jesus preached ?
It was a message of God’s good news brought first to the remnant of apostate Israel, mainly the tribe of Judah. Keep in mind the centuries of apostasy, intermixing philosophies and subjugation. Many things were lost and the Maccabees didn’t restore it all.