What is the Gospel?  The Gospel is:

concerning his Son, who was begotten by a seed of David according to the flesh and was appointed to be the Son of God in power according to the Spirit of holiness by his resurrection from the dead, Jesus Christ our Lord

There has been resistance to the idea that this is Paul’s summary of his own Gospel.  But consider Second Timothy 2.8:

Remember Jesus Christ, risen from the dead, the offspring of David, as preached in my gospel

There are some other parallels especially if we include Paul’s appeal to the “Old Testament.”  Compare Romans 1.1-4 to Acts 13.dd:

Paul, a servant of Christ Jesus, called to be an apostle, set apart for the gospel of God, which he promised beforehand through his prophets in the holy Scriptures, concerning his Son, who was begotten by a seed of David according to the flesh and was appointed to be the Son of God in power according to the Spirit of holiness by his resurrection from the dead, Jesus Christ our Lord

And we bring you the good news [Gospel] that what God promised to the fathers, this he has fulfilled to us their children by raising Jesus, as also it is written in the second Psalm,“‘You are my Son, today I have begotten you.”

While I don’t have a special argument that Paul wrote Herews, it begins in a much similar way to Romans:

Long ago, at many times and in many ways, God spoke to our fathers by the prophets, but in these last days he has spoken to us by his Son, whom he appointed the heir of all things, through whom also he created the world. He is the radiance of the glory of God and the exact imprint of his nature, and he upholds the universe by the word of his power. After making purification for sins, he sat down at the right hand of the Majesty on high, having become as much superior to angels as the name he has inherited is more excellent than theirs.For to which of the angels did God ever say,“You are my Son,today I have begotten you”?