Paul introduces himself as an apostle from Jesus. He wishes the church grace and peace from God.
No Other Gospel
Paul is amazed that the Galatian people are following a different gospel. He says there is no other true gospel and even if he came back and tried to teach them an alternate gospel they should be accursed.
I understand the idea of telling them not to follow anyone else preaching an alternate gospel, but to ban them from hearing an alternate gospel even from Paul just seems a bit confusing. What if Paul tells them something similar but different from what he told them before, is that a new gospel or a new take on the old one?
Paul Called by God
Paul did not learn the gospels he is teaching from anyone else, but instead from God through revelation. He used to persecute the church and tried to destroy it, but God spoke to him directly and he switched sides. He then spent three years away from all the other disciples and learned directly from God. Later people heard that the man who tried to destroy the church was preaching for it and they rejoiced in God because of me.
This claims he got revelation directly from God, how is someone supposed to tell the difference between that and simple insanity? Or if not insanity, how can you tell it isn't just your own thoughts instead of thoughts God has implanted in your brain?