Here's where we're at.
Paul had established his standing as an apostle. After he was called by the Risen Christ to repent of persecuting the church and follow him, Paul spent the next three years growing in his understanding of what it means to follow Jesus.
Everything Paul said and wrote was consistent with what Jesus taught during his three years of ministry. And Paul never contradicted himself nor changed the essential truth of the gospel he taught.
Paul was astonished that the Galatians had fallen under the spell of those who preached a false gospel. They were being told that following strict Jewish law was necessary for salvation. Paul taught the one, true gospel:
SALVATION BY GRACE ALONE THROUGH FAITH ALONE ON THE
BASIS OF CHRIST ALONE.
The false teachers were teaching something different. Paul could hardly believe it. As he writes in 1:6-7, "I am astonished that you are so quickly deserting him who called you in the grace of Christ and are turning to a different gospel - not that there is another one, but there are some who trouble you and want to distort the gospel of Christ."