I'm not claiming believing Gentiles are flawless, he's telling the Jewish believers they really need to rethink looking down on their believing Gentile brothers, bc God will judge based on actual righteousness, and will not excuse them just bc they're Jewish if they're lawbreakers/unrighteous, and will justify the doer of the Law even if they are Gentile.
I see the overall trend of the passage to conclude later in the next chapter, how Paul is demonstrating that all have sinned unto death, without exception. Those who think they are special because they have the Law aren't, because they don't keep it. Those who think they are special because they are Abraham's children aren't, because even the gentiles know right and wrong, and have an "inner law" to keep.
I see the heart of the passage where Paul wrote,
Romans 2:12-13 KJV
12) For as many as have sinned without law shall also perish without law: and as many as have sinned in the law shall be judged by the law;
13) (For not the hearers of the law are just before God, but the doers of the law shall be justified.
Whether you've been given the written Law, or whether you have the law in your heart, if you've sinned, you will be judged, and you will perish, no matter who you are, because it's the doers of the law who are just before God.
In "doers", this is absolute, and even 1 single sin renders you unrighteous, just as James wrote that to break a law makes you a lawbreaker. You can't subdivide the Law, it has to be kept in its entirety.
Romans 3:9-11 KJV
9) What then? are we better than they? No, in no wise: for we have before proved both Jews and Gentiles, that they are all under sin;
10) As it is written, There is none righteous, no, not one:
11) There is none that understandeth, there is none that seeketh after God.
So Paul concludes his indictment against humanity.
Romans 3:20-21 KJV
20) Therefore by the deeds of the law there shall no flesh be justified in his sight: for by the law is the knowledge of sin.
And forever puts out of reach the idea that anyone could ever be justified by good works. All the Law does, all it can do, is declare us guilty, as we all are.
21) But now the righteousness of God without the law is manifested, being witnessed by the law and the prophets;
So God has another way, and it doesn't involve the Law, except that Jesus fulfilled it on our behalf.
Much love!