Romans 15:17

‘I have therefore my glorifying in Christ Jesus in things pertaining to God.’

That is why he has something to glory of in the Messiah Jesus (Romans 9:1) in things pertaining to God, because his ‘offering up’ of Gentile believers won through his ministry has been successful and widespread, as the Messiah has wrought through him in his ministry (Romans 15:18).

We should note here that Paul is not seeking to exalt himself, but is rather seeking to lay down the basis of his authority for writing in the way that he has to the Church at Rome. He is presenting his credentials.