Was Paul qualified to speak with authority about the contents of the Gospel? After all, he wasn't one of the original twelve apostles. But Paul makes the claim here that he learned the gospel directly from Jesus Christ.
This is essential, though, if Paul's claim to be an apostle is to be taken seriously. The original twelve apostles learned the Gospel from Christ. In their cases, Christ was still present upon the earth, and taught them from His own lips. In Paul's case, Christ appeared to him in a blaze of resurrection glory (Acts 9:3). No one in the traveling party saw Christ but Paul (Acts 9:7). By this means Christ revealed Himself in His resurrection form to Paul.
Paul says he did not learn the gospel from men (Gal. 1:12). I gather from this statement that Paul only knew what his fellow Pharisees told him about Jesus, which would have been a tissue of lies. Believing the lies, Paul became enraged at this "cult leader" Jesus. He set out to exterminate Jesus' movement the way an oncologist would try to exterminate cancer cells in a body.
Paul later says that his behavior, though sinful, was also ignorant (1 Timothy 1:12-13), and for that reason the Lord took mercy upon him. This also implies that Paul knew nothing of the Gospel. He must have only known what other hostile witnesses told him.
So, considering that Paul had himself been victimized by a false message -- untrue accounts of who Jesus was and what Jesus did, which led him to commit blasphemous crimes against the Savior and His people -- he is very passionate about defending the integrity of the true Gospel. People who are uncaring about the integrity of the true Gospel have never suffered the evils that come from believing and acting upon a false one.
Paul was not a grade-B apostle; he wasn't a junior member of the team, or a player on the second string. It was crucial that the churches knew that, so that they bowed to his defense of the gospel as a word of divine authority. This is why he says he went off to Damascus to gather his wits. He didn't learn any of what he taught from the original twelve. He was not their disciple, he was their equal. We must respect Paul's writings for that reason.