An angel explained to Nephi that the Bible “is a record which is like unto the engravings which are upon the plates of brass, save there are not so many; nevertheless, they contain the covenants of the Lord, which he hath made unto the house of Israel; wherefore, they are of great worth unto the Gentiles” (1 Nephi 13:23, italics added). This can be viewed from at least a couple of perspectives.
If we speak of Gentiles as pure Gentiles, meaning those who are not descended from the twelve tribes of Israel, then the prophetic oracles of Isaiah are of worth to the Gentiles because many of the Gentiles will receive the gospel be adopted into the house of Israel, and thereafter enjoy fellowship with the Saints. Their destiny is thus the same as those who had been born into Israel or who had been members of the Church previously.