By “this cometh unto you,” Moroni means the entire Book of Mormon. Even though Moroni is writing in Ether, he understands that the whole work will come forth to the Gentiles in the last days. He addresses them directly, “O ye Gentiles.” He understands them, not only as his future readers but also as the dominant force when the Book of Mormon comes forth. Mormon wrote to the remnant of the house of Israel, but Moroni admonishes those to whom the promise is most relevant. The Gentiles must either serve God or be swept off the land.
Moroni compares them explicitly to “the inhabitants of the land” who have violated the promise and suffered the consequences (the Nephites). This aside to future readers has been inspired by the promise to the Gentiles, but it is fueled by Moroni’s personal experiences.