What exactly did he preach that was said to be gnostic? Thanks
This is a huge subject. It involves many of his words and phrases in his earlier works. His later works, which lack them, are suspected of being forgeries (possibly +Irenaeus). This is still being researched.
It is indeed strange that someone who did not know Christ in person would be elevated top be an Apostle of Christ by the Apostles -- yet the timing of him being placed in 'charge' of converting Gentiles comes at the time when Christianity as Judaism was dying rapidly.