Cranfield defies tradition and suggests that nature should be taken with what precedes. This would give the verse the following meaning; gentiles who do not possess the law by virtue of birth.
Romans 11:24 says For if you have been cut from what is by nature a wild olive tree. Nature here refers to gentile descent. Galatians 2:15 We ourselves are Jews by nature and not gentile sinners. Again, nature is used to denote physical descent. Ephesians 2:3 says And we were by nature children of wrath… and Romans 2:27 says Then those who are by nature uncircumcised. Paul obviously used this word, at least at times, in a physical and historical sense.
This fits into the argument of chapter 2 better. It is not the having or the hearing of the law that is important – it is the doing of the law. V10 has already established that a Greek / Gentile can be justified and saved by doing good. However, Paul is clear throughout chapters 4 & 5 that we are justified by faith alone, not by human effort. Hence this promise of salvation to those who do good, and who do not have the law cannot contradict with Romans 4 & 5. If these gentiles were not Christians, and they were doing what the law required why are they not saved? Paul is referring to gentile Christians who by their conversion have ‘done the law.’ Romans 13:8 says for he who loves his neighbor has fulfilled the law and Galatians 6:2 bear one another’s burdens, and so fulfill the law of Christ. The concept that Christians fulfill the law is certainly in Pauline writings. Hence a Christian can say they have done what the law requires through the grace and sacrifice that is in Christ.