ὅταν γὰρ ἔθνη … φύσει τὰ τοῦ νόμου ποιῶσιν

Says ὅταν with the present subjunctive ποιῶσιν is an indefinite temporal clause, "whenever (Gentiles) do." ἔθνη has no article: "Gentiles," not "the Gentiles" as a whole. What they do is τὰ τοῦ νόμου, "the things of the law" (BSB "what the law requires"), a partial and concrete phrase rather than "the law." The result is οὗτοι … ἑαυτοῖς εἰσιν νόμος, "they are a law to themselves."

Implies "A law to themselves" does not mean a law of their own making, but the law's demand present in them (2:15).

Says φύσει (physei) is an adverbial dative (Abbott-Smith: "by nature," Rom 2:14). Paul also uses φύσει for birth status (Gal 2:15, φύσει Ἰουδαῖοι; compare ἐκ φύσεως, Rom 2:27) and inborn condition (Eph 2:3).

Uncertain What φύσει modifies. (a) ποιῶσιν: they "do by nature what the law requires" (BSB). This note leans to (a), which suits ἑαυτοῖς εἰσιν νόμος. (b) τὰ μὴ νόμον ἔχοντα: Gentiles who "by birth" lack the law (compare Gal 2:15; Rom 2:27), the main alternative.

Uncertain Who these Gentiles are (L21(b)). (1) Gentiles generally, who at times do what the law requires (Calvin: "some notions of justice and rectitude"). (2) Gentile Christians, with the law written on the heart (Jer 31:33; Augustine, Spir. et litt. §§43–46, cf. §48; Cranfield and Gathercole, by reference). (3) A hypothetical case. This note leans to (1): φύσει goes with the verb, and the conscience of 2:15 mostly accuses. (2) is the main alternative. 2:14–15 and 2:26–29 need not describe the same group.