Verse 17 just says he went to Arabia, it doesn't say what he did there.
It is implied in verse 16, but I agree that it is ambiguous (as are many of his verses). That's why i say that reading +Paul is like eating fish with lots of bones — tiresome.
As to "his gospel", Paul says in verse 11 that the gospel he preached is not of man
"In the day when God shall judge the secrets of men by Jesus Christ according to my gospel." (Rom 2:16)
"Now to Him who is able to establish you according to my gospel and the preaching of Jesus Christ" (Rom 16:25)
Not only does he speak of his gospel, but his statements sound very Gnostic here.
[Kosta quoting:] "In the day when God shall judge the secrets of men by Jesus Christ according to my gospel." (Rom 2:16) [see also Rom. 16:25] Not only does he speak of his gospel, but his statements sound very Gnostic here.
Well, does it seem more reasonable to reconcile these verses by concluding that Paul made up his own gospel, apart from Christ, or that Paul is saying that his Gospel is identical to Christ's Gospel? If we choose the former, Paul would also be denying his own humanity. It seems pretty clear to me that Paul is saying that the Gospel he is preaching, "his Gospel", is the same Gospel that was given to him personally by Christ. Another way to look at it would be that if Paul did go out and preach his own made-up gospel, then it would mean that God made a colossal mistake in choosing Paul. :)