I believe it is true that the Orthodox have additional texts included in their Bibles. The reference to the gnostic gospel, the gospel of Judas shows their lack of understanding of the faith though. The Qur’an contains many of the stories contained in the gnostic gospels. These gospels are simply not true, and dangerously mis-leading. They are certainly not part of the deposit of faith for Christianity, nor are they the Word of God.
I’d have to say that it’s true that the Catholic Church is not the “exclusive preserver” of the Word of God. That’s not to say that the Catholic Church is the main force behind the preservation of the Word of God.