What does it mean that the Gentiles are a law unto themselves?
Being a law unto themselves means Gentiles who lacked Moses' Law still naturally did the things the Law requires, because the work of the Law was written in their hearts. Their conscience served as an internal moral standard, making them accountable and guilty of sin even without the Ten Commandments.
Les explains that when Gentiles do not have the Law of Moses, they can still by nature do the things contained in the Law. They are a law unto themselves, showing the work of the law written in their hearts, and their conscience bears witness. Even without the Ten Commandments—thou shalt not steal, kill, or commit adultery—they still have a moral system based on conscience. He illustrates this with Amazon Indians in loincloths who are uncivilized by outside standards but whose tribal community has a moral code. Where do they get it? Conscience. Scripture says that though they did not have the law, they were still guilty of sin just as much as anyone else.