D&C 20

As this revelation begins, speaking of the Prophet Joseph Smith, the Lord declares -

"...after repenting, and humbling himself sincerely, through faith, God ministered unto him by an holy angel, whose countenance was as lightning, and whose garments were pure and white above all other whiteness;

7 And gave unto him commandments which inspired him;
8 And gave him power from on high, by the means which were before prepared, to translate the Book of Mormon;
9 Which contains a record of a fallen people, and the fulness of the gospel of Jesus Christ to the Gentiles and to the Jews also;
10 Which was given by inspiration, and is confirmed to others by the ministering of angels, and is declared unto the world by them—
11 Proving to the world that the holy scriptures are true, and that God does inspire men and call them to his holy work in this age and generation, as well as in generations of old..."  (Doctrine and Covenants  Section 20:6 - 11)

It is in these words that the Savior states the record contains "the fulness of the gospel of Jesus Christ".  Some might interpret "fulness" to mean all that is to be revealed.  I feel a little bit different.  Fullness by definition is "the quality or state of being full".  The Savior taught -

"Now this is the commandment: Repent, all ye ends of the earth, and come unto me and be baptized in my name, that ye may be sanctified by the reception of the Holy Ghost, that ye may stand spotless before me at the last day.

21 Verily, verily, I say unto you, this is my gospel; and ye know the things that ye must do in my church; for the works which ye have seen me do that shall ye also do; for that which ye have seen me do even that shall ye do..."  (Book of Mormon   3 Nephi 27:20 - 21)

In my mind, the gospel of Jesus Christ is the doctrine and principles necessary to "stand spotless before me a the last day".  This is the good news and tidings of great joy that is to be published to the world for the Savior said that -

"...this gospel shall be preached in the whole world..."  (New Testament  Matthew 26:13)

The intent of the gospel then is that doctrine, principles, ordinances and the plan of salvation which will be preached to the "world. "  The fulness of the gospel would be full of these teachings, covenants, and ordinances.  Obviously this is not all of the "knowledge of God".  Then obviously there must be much more that may be revealed.  That which is taught in the temples, in my mind, is not the gospel that is to be taught to the world, but that which is to be taught in holy places to righteous individuals worthy of receiving greater knowledge and blessings.  This is my interpretation of the "fulness of the gospel".