Our last reference in Romans is found in Romans 16:25. Fittingly it is part of a prayer, a doxology:

25 Now to him who is able to establish you by my gospel and the proclamation of Jesus Christ, according to the revelation of the mystery hidden for long ages past, 26 but now revealed and made known through the prophetic writings by the command of the eternal God, so that all nations might believe and obey him- 27 to the only wise God be glory forever through Jesus Christ! Amen.

Paul’s gospel is the preaching of Jesus Christ – his life, his death and his resurrection. This gospel is something now revealed – a mystery now made known. This gospel involves including Gentiles by summoning them to faith and obedience.