Paul, a bondservant of Jesus Christ, called to be an apostle, separated to the gospel of God which He promised before through His prophets in the Holy Scriptures, concerning His Son Jesus Christ our Lord, who was born of the seed of David according to the flesh, and declared to be the Son of God with power according to the Spirit of holiness, by the resurrection from the dead. Through Him we have received grace and apostleship for obedience to the faith among all nations for His name, among whom you also are the called of Jesus Christ (Romans 1:1-6).
Immediately he links his apostleship with the gospel — he was 'called to be an apostle, separated to the gospel'. These aren't two unrelated callings, as he goes on to talk about both — the gospel and apostleship — as the greeting proceeds, drawing the two together again in verse 5 when he writes of 'grace and apostleship' received through Jesus.
So, Paul's apostleship is connected with his call to preach the gospel. In fact, Paul's apostleship is meaningless in separation from his call to preach the gospel — to be called as an apostle is to be separated to the gospel.