Many today have limited the Gospel of Jesus Christ to just those portions of Holy writ that specifically says the gospel i.e 1 cor 15:
1Moreover, brethren, I declare unto you the gospel which I preached unto you, which also ye have received, and wherein ye stand;
2By which also ye are saved, if ye keep in memory what I preached unto you, unless ye have believed in vain.
3For I delivered unto you first of all that which I also received, how that Christ died for our sins according to the scriptures;
4And that he was buried, and that he rose again the third day according to the scriptures:
Gal 3:
8And the scripture, foreseeing that God would justify the heathen through faith, preached before the gospel unto Abraham, saying, In thee shall all nations be blessed
but that is a mistake, yes those portions are indeed Gospel Truth, but they are not exclusively all that the gospel teaches. The Gospel of Jesus Christ or the Kingdom, is also the doctrinal Truths that we find in books of the bible like romans [chpt 1-11] and Ephesians, the first three chapters, for these are just as much as the gospel of Jesus Christ as 1 cor 15:1-4.
The mistake that many have made is that, they believe those doctrinal truths in those chapters were something new to the recipients, but they were not, for they were actually having the Gospel truths they had heard and received before, reiterated, or rehearsed again.
Thos letters were to strengthen and confirm them in what Gospel Truth they had previously heard.
Acts 14:
21And when they had preached the gospel to that city, and had taught many, they returned again to Lystra, and to Iconium, and Antioch,
22Confirming the souls of the disciples, and exhorting them to continue in the faith, and that we must through much tribulation enter into the kingdom of God.
Acts 18:32
And after he had spent some time there, he departed, and went over all the country of Galatia and Phrygia in order, strengthening all the disciples.
And so that was the reason for many of the letters or epistles, such as eph.
Lets look at eph 1 and see some of the Gospel Truth that Paul preached.
eph 1:
3Blessed be the God and Father of our Lord Jesus Christ, who hath blessed us with all spiritual blessings in heavenly places in Christ:
4According as he hath chosen us in him before the foundation of the world, that we should be holy and without blame before him in love
The Gospel Paul preached to Ephesus proclaimed : That the God and Father of our Lord Jesus Christ, has chosen a people for Himself.
And secondly, that He chose them before the foundation of the world was laid.
next, That He chose them that they should be Holy and without blame before Him in Love.
Next:
5Having predestinated us unto the adoption of children by Jesus Christ to himself, [When He chose us in Him]according to the good pleasure of his will,
6To the praise of the glory of his grace, wherein he hath made us accepted in the beloved.
In these few verses, Paul rehearses His Gospel and states:
Election or being chosen in Christ
The early date of this choice
The goal or purpose of this choice, to be holy and without blame before Him in Love.
The predestination of them chosen, and what they had been predestined to,
And that they had been made accepted to God, in the Beloved.
Now is not that good news ?