The fulness of the Gospel is the things we need on this earth (authority through the proper priesthood, ordinances, scriptures, etc.) for our salvation. Fulness does not mean the complete, entire knowledge and wisdom that God has. Although we have the fulness of the Gospel for our salvation - we need additional light and knowledge to help us and guide us.  
 
The same analogy could be given with the New Testament. Surely Jesus taught the fulness of the gospel. So why do we have the teachings of the Apostles after Jesus was crucified? It is because we always need light and truth to help us. We always want help from Our Heavenly Father in whatever situation or circumstances we may find ourselves.