Paul received the Gospel directly from Jesus Christ, it was the same Gospel that Peter preached on the day of Pentecost, but with much more detail, Galatians 1:11, 12.
Why is it that you cannot believe your Jewish brothers?
Because they had nothing to do with the NT. They were Jewish all right but they would never write a page of the NT. They were used by Paul and the Church later to borrow apostolic credibility for the church of Paul. BTW, after listening to their gospel preaching, Paul got disappointed that they seemed to be preaching a different gospel about a different Jesus and for that matter, Paul would consider them as false apostles disguised as apostles of Jesus. (II Cor. 11:4-6, 13)