The Gospel in Romans

Paul introduces the gospel in the opening verses of Romans:

1 ¶ Paul, a bond-servant of Christ Jesus, called as an apostle, set apart for the gospel of God,
2 which He promised beforehand through His prophets in the holy Scriptures,
3 concerning His Son, who was born of a descendant of David according to the flesh,
4 who was declared the Son of God with power by the resurrection from the dead, according to the Spirit of holiness, Jesus Christ our Lord,

The gospel is about Jesus, a descendant of David and the Son of God as well. The gospel was promised in the OT, through the prophets.

This message about Jesus Christ, son of David and son of God has the power to save those who believe:

15 So, for my part, I am eager to preach the gospel to you also who are in Rome.
16 ¶ For I am not ashamed of the gospel, for it is the power of God for salvation to everyone who believes, to the Jew first and also to the Greek.