A perversion of the true gospel will rob the gospel of its power. They were preaching law and outward obedience as the means of salvation. They didn't "get it", just as we often don't get it today.
Gospels today can get twisted just as they were in Galations. Well intended actions may stress certain acts/attitudes/beliefs and they can be misplaced in terms of importance. The truth of the gospel is simple, yet it is profound. It is hard for the human intellect to accept. We are conditioned to "earn" our way. Thus, misappropriated efforts are spent on trivial matters and the souls of the children are corrupted.