No the Bible doesn’t contain all the word of God, only the portion that God wanted preserved. And it was preserved in just the way he wanted. He sent us the Holy Spirit to lead us to all truth.
Gospel means Good News. The Bible is perfectly adequate in giving us all the good news that we need.
1 We are set free from the penalty of sin which is death through the atonement of Christ. </ end gospel>
THAT is Good News. Finding out there are additional works we must perform is not Good News. Finding out that the author of these additional works is a fraud, is definitely NOT good news.
And we hold that God does indeed preserve his word that he wants preserved. The plates of Nephi teach us that indeed. Cheers, not I'm really off.