Full of what?

The Book of Mormon supposedly contains the "fulness of the Gospel," but what exactly does that mean? For apologists it tends to mean "the good tidings of Jesus' suffering for our sins and his conquering of death" (as though the Creator of the Universe hadn't already proven his power to give life!),  but I would argue that the term as it is actually used in the LDS Church means something more like "everything we need to know and do to reach the Celestial Kingdom where we will be with Heavenly Father and our eternal families." The second definition is encouraged every time the general Church authorities speak of "living the Gospel" and include a number of other doctrines, rites, and behaviors that have nothing to do with faith, repentance, baptism, and receiving the Holy Ghost. Besides, faith, repentance, baptism, and receiving the Holy Ghost are just the first four principles and ordinances, not the complete list.

Mormons know that living the Gospel fully means regular Church attendance, fulfilling service callings in the Church, paying tithing, and doing everything required in the temple, which just happens to include marriage sealings. The Family proclamation states that "Marriage between man and woman is essential to His eternal plan" which sounds a lot like the way we talk about faith, repentance, baptism, and receiving the Holy Ghost - the are essential. Not only that, having children and continued posterity, as it turns out, is definitely considered a key component to living the fullness of the Gospel. The Mormon Gospel is not merely that Christ made restitution and resurrection possible for all of us mortal sinners, it's the promise of eternal families who will progress together to godhood. This is the "good news" of Christ that the Church is selling.

Of course, you have to get this from the missionaries and other members exclusively because relying solely on The Book of Mormon will leave the Gospel of Mormonism feeling dated and void.