Earlier in this letter, he had reminded them that freedom is not free, that Christ had paid a price for their freedom (1:3-5). And since, they had received this gift, they should not reject it (1:6-12).
These Galatians don't have to become Jews in order to be Christians. But, in no way, does Paul ever indicate that anything goes.
If we interpret the word "law" to be mean "male Christians have to be circumcised," then Paul is saying they aren't bound by the law. But, Paul doesn't always use the term "law" that narrowly.
In the sense that they--we, too--are supposed to consider that the law was intended as a way to show the people how to live in a way that would be best for all of them, a way that would promote peace and continuity, then we are to follow the law.
And, in that case, Christians are still bound by the law. "Do not use your freedom as an opportunity to indulge yourself," Paul tells them. He quotes Leviticus 19:18 to demonstrate that the law can be summed up in a single commandment, "Love your neighbor as yourself."
Love not as some sweet emotion but love taken seriously. "Through love become slaves to one another."