[And this cometh unto you]: “This” is the entirety of the Book of Mormon. Even though Moroni is currently writing in Ether, he understands that it is the whole of the work that it to come forth to the gentiles in the last days.
[O ye Gentiles]: This is a specific reference to the future readership of the Book of Mormon. However, it is a reference that carries with it the prophetic understanding that when then Book of Mormon comes forth it will be the Gentiles who are in control of the land. Mormon had written to the remnant of the house of Israel, but at this point Moroni’s admonition comes directly to those who are most affected by the nature of the promise of the land. The Gentiles will be in possession of it, therefore they are under the requirement of the promise; to serve God or be swept off the land.
[as the inhabitants of the land have hitherto done}: Moroni makes his comparison explicit. This aside to the future readers has been inspired by the promise to the Gentiles, but it is fueled by Moroni’s personal experiences.