Is the Talmud saying that the book of Job speaks primarily to Gentiles, or is it referring to other unwritten prophecies of Job and the others? If its referring to the Book of Job's words, in what way does the book of Job speak more to Gentiles than to Jews?
The Books of Obadiah, Nahum, and Jonah contain prophecies that are predominately addressing Gentiles. The Talmud does not mention them as prophets of the Gentiles though, only Beor, Bilaam, Job, and his 4 companions.
So perhaps the Talmud is referring to the general trend of their prophecy, most of which remained unwritten (Megillah 14b), not specifically to their written works. Obadiah, Nahum, and Jonah mostly prophesied for Israel. The Talmud's 7 mostly prophesied for the Gentiles. We don't see the bulk of their prophecy because it wasn't eternally applicable and didn't need to be written down.
Alternatively, The Talmud could simply be stating to whom the prophecy was said, and not about whom it was said, or for whom it was most pertinent. So the Book of Job, while codified by Moses (B Bath 14a), was originally prophesied by Job and his friends to Gentiles.
This approach finds merit upon examining Bilaam's prophecy (Num 23, 24), which contained much pertaining to Israel, and may have been his only prophetic experience (Nahmanides on Num 22:31), yet Bilaam is listed by the Talmud.