Romans 2.6:

So, if a man who is uncircumcised keeps the precepts of the law, will not his uncircumcision be regarded as circumcision?

Romans 4.9-11:

Is this blessing then only for the circumcised, or also for the uncircumcised? We say that faith was counted to Abraham as righteousness. How then was it counted to him? Was it before or after he had been circumcised? It was not after, but before he was circumcised. He received the sign of circumcision as a seal of the righteousness that he had by faith while he was still uncircumcised.

Still chewing on this.  Plainly, the “law” was not given until Sinai.  But Paul also speaks of “precepts of the Law” and Gentiles becoming a “law to themselves” (2.14).  Further, Paul describes Abraham’s faith as praiseworthy:

16 That is why it depends on faith, in order that the promise may rest on grace and be guaranteed to all his offspring—not only to the adherent of the law but also to the one who shares the faith of Abraham, who is the father of us all, 17 as it is written, “I have made you the father of many nations”—in the presence of the God in whom he believed, who gives life to the dead and calls into existence the things that do not exist. 18 In hope he believed against hope, that he should become the father of many nations, as he had been told, “So shall your offspring be.” 19 He did not weaken in faith when he considered his own body, which was as good as dead (since he was about a hundred years old), or when he considered the barrenness of Sarah’s womb. 20 No distrust made him waver concerning the promise of God, but he grew strong in his faith as he gave glory to God, 21 fully convinced that God was able to do what he had promised. 22 That is why his faith was “counted to him as righteousness.”

While this justification is available for David when he sins as well (though if you read the rest of his Psalm it gets interesting), the fact remains that trust in the Lord is itself the real keeping of the Law–because it is loyalty to God/Jesus which God promises to forgive and exalt and to which applies the representative headship of Jesus with the imputation of righteousness, it is commanded by God and thus is obedience, and it is the motivational basis for following God in all his ways.  Thus Romans 3.31: “Do we then overthrow the law by this faith? By no means! On the contrary, we uphold the law.” And 3.27: “Then what becomes of our boasting? It is excluded. By what kind of law? By a law of works? No, but by the law of faith.”  Or from Romans 9.31-32: “Israel who pursued a law that would lead to righteousness did not succeed in reaching that law. Why? Because they did not pursue it by faith, but as if it were based on works.”

So even though it is not technically law, it does seem that Abraham is the example of the Gentile who is regarded as circumcised because he trusts in God.  Hebrews 11 also points out how faith works itself out in love (Galatians 5.6):

8 By faith Abraham obeyed when he was called to go out to a place that he was to receive as an inheritance. And he went out, not knowing where he was going. 9 By faith he went to live in the land of promise, as in a foreign land, living in tents with Isaac and Jacob, heirs with him of the same promise. 10 For he was looking forward to the city that has foundations, whose designer and builder is God. 11 By faith Sarah herself received power to conceive, even when she was past the age, since she considered him faithful who had promised. 12 Therefore from one man, and him as good as dead, were born descendants as many as the stars of heaven and as many as the innumerable grains of sand by the seashore.

13 These all died in faith, not having received the things promised, but having seen them and greeted them from afar, and having acknowledged that they were strangers and exiles on the earth. 14 For people who speak thus make it clear that they are seeking a homeland. 15 If they had been thinking of that land from which they had gone out, they would have had opportunity to return. 16 But as it is, they desire a better country, that is, a heavenly one. Therefore God is not ashamed to be called their God, for he has prepared for them a city.

17 By faith Abraham, when he was tested, offered up Isaac, and he who had received the promises was in the act of offering up his only son, 18 of whom it was said, “Through Isaac shall your offspring be named.” 19 He considered that God was able even to raise him from the dead, from which, figuratively speaking, he did receive him back.

And this corresponds to how Abraham is described in Genesis 18:

17 The Lord said, “Shall I hide from Abraham what I am about to do, 18 seeing that Abraham shall surely become a great and mighty nation, and all the nations of the earth shall be blessed in him? 19 For I have chosen him, that he may command his children and his household after him to keep the way of the Lord by doing righteousness and justice, so that the Lord may bring to Abraham what he has promised him.”

And also in Genesis 26.5 “Abraham obeyed my voice and kept my charge, my commandments, my statutes, and my laws.”

Now, neither of these makes a distinction between his pre- and post-circumcision faith resulting in obedience, so Paul singles out what happens before circumcision.  But it is hard not to see Abraham, whom Paul singles out as a Gentile for a time, despite the fact that the Law is not yet given, is supposed to count as proof of Paul’s rhetorical question, “So, if a man who is uncircumcised keeps the precepts of the law, will not his uncircumcision be regarded as circumcision?”