Galatians 1:6 & 7

"I am astonished that you are so quickly deserting the one who called you by the grace of Christ and are turning to a different gospel - which is really no gospel at all. Evidently some people are throwing you into confusion and are trying to pervert the gospel of Christ." NIV translation

Paul gets right to the heart of the matter as he says that the believers have abandoned the gospel and accepted a fake. There were people among the believers that wanted to add works to grace to get salvation. We, as people, have a tendency to want to do something to get something but, if you add works for salvation then it is no longer a gift and grace.

Galatians 1:8 & 9

"But even if we or an angel from heaven should preach a gospel other than the one we preached to you, let him be eternally condemned! As we have already said, so now I say again: If anybody is preaching to you a gospel other than what you accepted, let him be eternally condemned!" NIV translation

Paul says the same thing here twice and we should remember that the number two is the number of witnesses. He warns them that no matter who trys to tell them a different way to be saved (even if it was an angel from heaven) they must be ignored. Those that preach another gospel are eternally condemned and that may sound like strong words and judging someone's salvation. The simple fact is that Jesus said that He was the only way to be saved and God confirmed it with Jesus' resurrection. Those that preach anything but the free gift of salvation through Jesus Christ stand condemned already because they do not believe that Jesus' sacrifice was sufficient and there is no other way.