“Paulus, ausgesondert … für das Evangelium Gottes…” (Römer 1,1)
The phrase Pauls uses involves a part of speech in the Greek language called the Genitive, which indicates possession. He is not saying, ‘I have been commissioned to announce a message or good news about God.’ Rather, he is saying that the Gospel he has been separated and called to proclaim is God’s Gospel. God is the autor and owner of it. Paul is simply the messenger whom God has called and set apart to proclaim to people a message that comes from God himself.
… (T)he phrase ‘the Gospel of God’ did not mean the Gospel about God but, rather, the Gospel that is the Possession of God. God owns that Gospel. He is the one who invented the Gospel and commissioned Paul to teach it. The Gospel did not originate with Paul; it originated with God.
R. C. Sproul. St. Andrew’s Expositional Commentary. Romans. S. 18+27.