Shimon; I understand whaly you see that in the gemara, and you got me thinking about it, but I think this conclusion is untenable in light of chazal and rishonim, as I will explain. The rambam below in melachim 8:11 is very clear that only goyim who are mekabel the 7 mitzvos have olam haba.
כל המקבל שבע מצות ונזהר לעשותן הרי זה מחסידי אומות העולם. ויש לו חלק לעולם הבא. והוא שיקבל אותן ויעשה אותן מפני שצוה בהן הקב”ה בתורה והודיענו על ידי משה רבינו שבני נח מקודם נצטוו בהן. אבל אם עשאן מפני הכרע הדעת אין זה גר תושב ואינו מחסידי אומות העולם ולא מחכמיהם:
There is a machlokes in sanhedrin 105 between rebbe Eliezer and rebbe yehoshua if even a goy who keeps the 7 mitzvos has olam haba, and the rambam is paskening like rebbe yehoshua that they do. Interesting to note that shu”a paskens like rebbe Eliezer, because in hilchos gerim 268:2, where we tell the prosective ger once he has been accepted that –
ואומרים לו הוי יודע שהעולם הבא אינו צפון אלא לצדיקים והם ישראל
In light of this, the gemara in rosh hashana seems to be going like rebbe yehoshua – but rebbe yehoshua only grants olam haba to goyim who keep the 7 mitzvos! How then can poshei umos haolam have any gan eden?
My understanding is that the lashon of the gemara is very meduyak; it doesn’t say poshei akum, rather umos haolam – I think we’re talking about gerei toshav who accepted the mitzvos, but are sinners who have done bad things that do not include avodah zara; maybe they’ve eaten aiver min hachai, or other things. The same way a jew who sins occasionally has not forfeited olam haba, but rather one who routinely flouts halacha is included in poshei yisroel….poshei umos haolam may refer to a casual non-jewish sinner.
Also, logically, does it make sense that Hashem would take away the olam haba of a goy who generally does good but sometimes stumbles?
It would also be meduyak to put them together, because (i have to fet the mareh makom) a later chabad sefer writes that goyim who keep the 7 mitzvos have the same shoresh neshoma as a yid(not the same neshoma, just the shoresh), so it makes sense that the poshei yisroel and the non-jewish sinner will end up in a similar place.