Eschatology: Guaranteed Triumph (Genesis 3:15)

The Lord made very clear to Adam and Eve that if they fail to protect or obey the Lord they will die. We find that they do in fact fail, they do eat from the tree of the knowledge of good and evil, but they still walk out of the garden of Eden. Did the Lord make a false threat? What is death and the intention of death? Is the Gospel promised only through the prophets or do we find a Gospel promise earlier in Scripture?