Avodah Zarah 3A

לומר, שאף על פי שמקיימין אותן אין מקבלין עליהם שכר.

This serves to say that even if they fulfill the 7 Noahide mitzvot they do not receive a reward for their fulfilment.

ולא? והתניא: היה רבי מאיר אומר: מנין שאפילו גוי ועוסק בתורה שהוא ככהן גדול? תלמוד לומר: ״אשר יעשה אתם האדם וחי בהם״, כהנים לוים וישראלים לא נאמר, אלא ״האדם״, הא למדת שאפילו גוי ועוסק בתורה הרי הוא ככהן גדול.

The Talmud asks: And are they not rewarded for fulfilling those mitzvot? But isn’t it taught in a baraita that R' Meir would say: From where is it derived that even a non-Jew who engages in Torah study is considered like a High Priest? The verse states: “You should therefore keep My statutes and My ordinances, which if a person do, and should live by them” (Leviticus 18:5). It is not stated: Priests, Levites, and Israelites, but rather the general term “person.” From here you learn that even a non-Jew who engages in the study of Torah is like a High Priest. This demonstrates that non-Jews are rewarded for fulfilling mitzvot, despite the fact that they are not commanded to do so.

אלא לומר לך, שאין מקבלין עליהם שכר כמצווה ועושה, אלא כמי שאינו מצווה ועושה, דאמר רבי חנינא: גדול המצווה ועושה יותר משאינו מצווה ועושה.

Rather, the verse serves to tell you that they do not receive as great a reward for their fulfillment as one who is commanded and performs a mitzva. Rather, they receive a lesser reward, like that of one who is not commanded and still performs a mitzva. As R' Ḥanina says: Greater is one who is commanded to do a mitzva and performs it than one who is not commanded and performs it.

אלא כך אומרים הגוים לפני הקדוש ברוך הוא: רבונו של עולם, ישראל שקיבלוה היכן קיימוה?

The Talmud returns to the discussion between God and the non-Jewish nations, whose claims are rejected with the rebuttal that they did not receive the Torah because they did not fulfill the 7 Noahide mitzvot that were incumbent upon them. Rather, this is what the non-Jews say before God: God! as for the Jewish people who accepted the Torah, where is the evidence that they fulfilled its mitzvot?