Question: Does the Book of Mormon contain the “fulness of the gospel of Jesus Christ”?
Answer: The word gospel in Greek means “good news.” The word gospel also means “God’s story” and “good tidings.” Using these definitions, the Book of Mormon contains the fulness of the gospel.
Scholar Daniel H. Ludlow added another definition. He wrote, “The fulness of the gospel means that it contains those instructions a person needs to observe in order to be worthy to enter the presence of God in the celestial kingdom.”
Source: 400 Questions and Answers About the Book of Mormon by Susan Easton Black, p 43.