"Rabbi Meir would say: ‘From where is it derived that even a non-Jew who engages in Torah study is considered like a High Priest? It is derived from that which is stated: “[You shall therefore keep My laws and My decrees,] which if a man does he shall live by them” (Vayikra 18:5) Sanhedrin 59a
What did Rabbi Meir mean by this statement? Is he talking specifically about the 7 laws?
Rabbi Meir is talking about the merit of non jews studying Torah, meaning, the Sheva Mitzvot bnei Noach. The conclusion of that is in the continuation of Gemara in Sanhedrin 59a which you quote:
התם בשבע מצות דידהו
There (in the baraita) [the reference is to] their Sheva Mitzvot.
R. Yaakov b. R. Abba Mari Anatoli (13th century) in his Malmad haTalmidim (parshas Toldos) elaborates:
ומוקיס לה בגמרא בשבע מצות דידהו והוא הנותן לכל למוד חכמה הצריכה לאמת עיקר המצות ההן ועיקר המצות שבתורה
The Gemara understands this as regarding to their Sheva Mitzvot and that also applies to their study of any wisdom for stablishing the truth of these mitzvot or the fundaments of [their] mitzvot in the Torah.
