Paul's epistles were written before the gospels.
The truth about the God of truth is in the Bible, which was inspired by the Spirit of truth.
But you won't accept that. You reject that Jesus' chosen instrument was inspired by the Holy Spirit, or maybe sometimes, but mostly not, yet you claim that the Holy Spirit inspires you to read books that contain the truth also but which were not included in Scripture.
Well if you're prepared to tell the Lord one day that his chosen man was "tickling people's ears" and preaching "goodies" and "sugar coated teachings"; go ahead. It is entirely inconsistent to say that Paul was chosen by the Lord Jesus and a saint and you accept some of Paul's teachings, but that he was also a false apostle, arrogant and a man with his own agenda.
If he was false you can't trust him; yet he, an untrustworthy man, was chosen and called by Jesus. If he was false, then the Holy Spirit shouldn't have led people to put his letters in the Bible. If he was false, then the churches wouldn't believe him, and anyone who read his words would be led astray, confused or damned - because the thief comes to steal and destroy. Yet thousands are edified by his writings which have brought life, hope and truth to us.
Yes, but your definition of the Gospel is the words of Jesus. You seem to say that it is the words of Jesus, alone, that save - and anyone who wasn't around to hear them when they were spoken, is false and not preaching the Gospel. That is incorrect. We are saved from our sin by the cross, not by the sermon on the mount. You won't accept that view, but it is truth and orthodox Christian doctrine.