J. Vernon McGee Commentary Gal 1:6:

Now the subject of Paul’s letter to the Galatian believers concerns the interpretation of the facts of the gospel. The Judaizers had followed Paul into the Galatian country. They did not challenge the facts of the gospel. After all, five hundred people at once saw the Lord Jesus after His resurrection. When you have that many people around as witnesses, you don’t run around denying the facts of the gospel. The heresy they were promoting concerned the interpretation of those facts. They were very sly and subtle and said something like this, “Did Brother Paul come here among you?” The folk would say, “Yes, he came and preached the gospel and we accepted it. We are converted. We know Christ as our Savior, and we are in the body of believers.” The Judaizers would respond, “Oh, that’s wonderful. Brother Paul is accurate as far as he goes, but he doesn’t go far enough. Did he tell you that you should keep the Mosaic Law? Oh, he didn’t? Well, he should have told you that. Yes, you are to trust Christ, but you must also follow the Mosaic Law or you won’t be saved.”
This is one of the oldest heresies known, and it is still with us today. It is adding something to the gospel of grace; it is doing something rather than simply believing something. It is faith plus something rather than faith plus nothing. Every cult and “ism” has something for you to do in order to be saved.
It is interesting that Paul said to the Philippian jailer, “… Believe on the Lord Jesus Christ, and thou shalt be saved …” (Acts 16:31). Simon Peter said to the Sanhedrin, “Neither is there salvation in any other; for there is none other name under heaven given among men, whereby we must be saved” (Acts 4:12). Christ told the apostles to preach the gospel of salvation by grace. They were not to do anything to gain their salvation, but they were to trust what Christ already had done for them. The gospel shuts out all works.
Now Paul is writing to the Galatian believers and saying, “I marvel that ye are so soon removed from him that called you into the grace of Christ unto another gospel”—
McGee, J. Vernon: Thru the Bible Commentary. electronic ed. Nashville : Thomas Nelson, 1997, c1981, S. 5:153