What does Galatians 1:8 mean?

Paul has expressed his great astonishment that the Galatian Christians, who had so profoundly come to Christ with great rejoicing when he brought the gospel message to them, were now abandoning Christ to believe a distorted "gospel."

The distortion was this: A group of religious Jews were going to places where non-Jews—referred to as Gentiles—had believed Paul's preaching and received salvation from their sin by trusting in Christ's death for that sin on the cross. These Judaizers could not stand the idea that these Gentiles believed themselves right with God without following the law of Moses. So they came teaching that true salvation required both faith in Christ and obedience to the Law.

Astoundingly to Paul, some of these Galatian Christians believed them! Paul now curses anyone who would teach a version of the gospel of Jesus other than the one they heard from him. He writes that this includes himself or even an angel from heaven. If "the gospel" is modified, adjusted, or changed by anyone for any reason, it is no longer the gospel. It is not the truth.