3 Nephi 29 – 30
I think that chapter 30 is the only chapter written specifically to the Gentiles. And what is chapter 30 – a direct invitation to repent and come unto Christ and be numbered among the house of Israel.
It is interesting to think that in 3 Nephi 28 we have read the last doctrine in the Book of Mormon that was not written by Mormon or Moroni. Prior to this they could copy portions from the existing record, but from here on they are either writing original work or translating for the Book of Ether.