That they were removed to another gospel, which yet was not another. Thus the apostle represents the doctrine of these judaizing teachers; he calls it another gospel, because it opened a different way of justification and salvation from that which was revealed in the gospel, namely, by works, and not by faith in Christ. And yet he adds, “Which is not another-you will find it to be no gospel at all-not really another gospel, but the perverting of the gospel of Christ, and the overturning of the foundations of that”-whereby he intimates that those who go about to establish any other way to heaven than what the gospel of Christ has revealed are guilty of a gross perversion of it, and in the issue will find themselves wretchedly mistaken. Thus the apostle endeavours to impress upon these Galatians a due sense of their guilt in forsaking the gospel way of justification; and yet at the same time he tempers his reproof with mildness and tenderness towards them, and represents them as rather drawn into it by the arts and industry of some that troubled them than as coming into it of their own accord, which, though it did not excuse them, yet was some extenuation of their fault. And hereby he teaches us that, in reproving others, as we should be faithful, so we should also be gentle, and endeavour to restore them in the spirit of meekness.

Matthew Henry, Commentary on Galatians 1:6-9