He makes it clear:
Romans 2:13-15
King James Version (KJV)
13 (For not the hearers of the law are just before God, but the doers of the law shall be justified.
14 For when the Gentiles, which have not the law, do by nature the things contained in the law, these, having not the law, are a law unto themselves:
15 Which shew the work of the law written in their hearts, their conscience also bearing witness, and their thoughts the mean while accusing or else excusing one another)
"Those who "hear the Law" would be those hearing the written Law, which cannot be divorced from either the Covenant or the principles written therein. So the "Law" in view would be first and foremost the Hebrew Scriptures (and we will see primarily a reference to the Pentateuch), though we can view this as "God's will for man as recorded by the Writer/s." The Hebrew Scriptures were divided by the Lord into three parts in the mind of the Jew, and this is maintained by the Lord Himself:
Luke 24:44-45
King James Version (KJV)
44 And he said unto them, These are the words which I spake unto you, while I was yet with you, that all things must be fulfilled, which were written in the law of Moses, and in the prophets, and in the psalms, concerning me.
45 Then opened he their understanding, that they might understand the scriptures,
We see a division also mentioned by the Lord here:
Luke 16:27-29
King James Version (KJV)
27 Then he said, I pray thee therefore, father, that thou wouldest send him to my father's house:
28 For I have five brethren; that he may testify unto them, lest they also come into this place of torment.
29 Abraham saith unto him, They have Moses and the prophets; let them hear them.
Note that the Lord's teaching has Abraham saying the brothers must "hear" Moses and the Prophets, which would refer to the Pentateuch (first five Books of Scripture credited to Moses) and the Books of Prophecy, which are distinguished from the Wisdom Books (i.e., Song of Solomon, Ecclesiastes, Psalms, and Proverbs, for example).
So when we go back to our text...
Romans 2:13-15
King James Version (KJV)
13 (For not the hearers of the law are just before God, but the doers of the law shall be justified.
14 For when the Gentiles, which have not the law, do by nature the things contained in the law, these, having not the law, are a law unto themselves:
15 Which shew the work of the law written in their hearts, their conscience also bearing witness, and their thoughts the mean while accusing or else excusing one another)
...the "Law" in view would primarily be the Pentateuch, because we would probably attribute a consistency to Paul in regards to terminology, and, we see a specific statement concerning the works of the Law being performed by Gentiles who had never "heard" the Law.
Secondly, we have to look at what that itself means (performing the works of the Law written on the hearts of men who had never heard either the Written Law or were exposed to the Covenant of Law). Basically, in view are the principles found in the Law which express one thing: God's will for mankind. So these Gentiles, though never having heard the Law, obeyed those principles which were internal, written on their hearts. Paul is basically making the point that it is within man's nature to know the principles of God's will (i.e., not stealing, lying, committing adultery, etc.) even though one has not been exposed to the Word of God.
We do not, though, make that a basis for Free Will in a salvific context, because we would then violate what is also taught in the Law as well as the New Testament, that no man seeks after God. We also do not equate a Gentile obeying those principles with one obeying the Gospel of Jesus Christ, or that those men were saved by their obedience. Justified, perhaps, as pertaining the flesh, but they will all stand before Christ and be judged according to their response to the Law written on their hearts:
Romans 2:15-16
King James Version (KJV)
15 Which shew the work of the law written in their hearts, their conscience also bearing witness, and their thoughts the mean while accusing or else excusing one another)
16 In the day when God shall judge the secrets of men by Jesus Christ according to my gospel.
So it is my opinion that these Gentiles will stand at the Great White Throne and be judged according to the Books (of the written Word). They were not made regenerate, as that did not begin until Christ came, and a failure to make that distinction confuses many when it comes to the question of whether "we have to keep the Law" or not. The Law is part of the Covenant of Law, which some do not realize was given distinctly as a means of relationship between God and the People He created to be in relationship with Him, Israel. It was not, as the New Covenant is, given for "all families of the earth," and even though one could become a proselyte that did not make them a part of Israel to where they enjoyed everything Israel did.
There are no Proselytes in Christianity. All who enter into New Covenant Relationship are made one with God and each other. There is no distinction between Jew and Gentile.
And we do not keep the Law to be saved, though we certainly, as Paul wrote, establish the Law. Meaning...we perform the works of the Law. Or in other words, we live according to those same principles given in the Law, and do not come into conflict with them. That is because God promised that would take place...
Ezekiel 36:27
King James Version (KJV)
27 And I will put my spirit within you, and cause you to walk in my statutes, and ye shall keep my judgments, and do them.
God bless.