It is not just the non-observance of particular laws that Paul is inveighing against in Galatians [3.10-14]. Nor is his argument against ‘works’ or ‘doing’ in general. … in Gal. 5-6 Paul is very happy to insist on the importance of various sorts of works and deeds, indeed on obedience to ‘the Law of Christ.’ Paul’s argument is against observance of a specific kind of works – works of the Mosaic Law in general.
The quotation of Deuteronomy makes clear enough that the curse falls on all those who are under the Law and do not remain in all of it so as to do all of it. This curse has nothing to do with general human fallenness but more specifically with being a Mosaic law breaker, and thus it applies only to those who are ‘under the Law.’ If the Galatians submit to the Mosaic Law they will also indeed be subject to such a curse, if that is, they fail to keep all the Law.
Grace in Galatia by Ben Witherington; pp. 232-233