Post History

The g'mara takes this up on [105a](https://www.sefaria.org/Sanhedrin.105a.10?lang=bi&with=all&lang2=en).  The discussion in the g'mara suggests that gentiles *can* enter the world to come:

> [The mishna] is in accordance with the opinion of Rabbi Yehoshua, as it is taught in a baraita that Rabbi Eliezer says: It is written: “The wicked shall be turned back to the netherworld, all that nations that forget God” (Psalms 9:18). 
>
> “The wicked shall be turned back to the netherworld”; these are the sinners of the Jewish people, as only the sinners are sentenced to the netherworld. “All the gentiles that forget God”; these are the sinners of the gentiles. From the fact that it is written: “All the gentiles,” it is apparent that none of the gentiles have a share in the World-to-Come. This is the statement of Rabbi Eliezer. 
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> Rabbi Yehoshua said to him: But is it stated in the verse that the sinners of the Jewish people will be like all of the gentiles? It is stated only: “All the gentiles that forget God.” Rather, the wicked shall be turned back to the netherworld, and who are they? They are all the gentiles that forget God. Gentiles who fear God do have a share in the World-to-Come. (Translation from Sefaria)

If no gentiles have a share in the world to come (per R' Eliezer), then it is unclear why the mishna would list specific gentile exclusions.  If, on the other hand, R' Yehoshua is correct, then some gentiles *do* have a share in the world to come, and it makes sense to list gentile exclusions alongside Jewish exclusions.

The mishna contradicts its first statement even before we get to Bilaam, by saying that "all Yisrael" have a share and then going on to talk about some who do not.  Given the "rambling associativeness" style of the talmud, I'm not surprised that it then continues on to gentiles who do or don't have a share without explicitly introducing that category first.