Galatia was a region of modern Turkey where Paul had planted churches. Many of the Jews in those churches had mistaken the Gospel for a new version of Judaism. So their Gospel went something like this:
Jesus + Law + Circumcision = Eternal Life
Paul's problem with this, and it should be our problem too, is that nothing can be added to the message of the Gospel of Jesus. He's not who we want Him to be, He is who He is, He said what He said, He did what He did. Paul makes the argument throughout Galatians that the Gospel is:
Jesus = Eternal Life
You can't add anything to Christ, otherwise His work was insufficient. He died on the cross but it wasn't enough to satisfy the breach between God and humanity. So, summarizing, Paul argues that "so we have also have believed in Christ Jesus in order to be justified by faith in Christ and not by the works of the law, because by the works of the law no one will be justified" (2.16).
That really is Good News.