The Apostles decided upon the composition, and of course they wrote (or dictated) most of the books of the New Testament.
No merely old book gets to be a Gospel or part of the Bible: it has to have been accepted by the authority of the Church.
There are various gnostic ‘gospels’ - they are not actual Gospels. They (for instance) contain crazy stories about the young Christ turning his friends into bread and baking them in an oven. As you can guess: the gnostic ‘gospels’ are made-up nonsense and have no validity.
I think your post is the way to look at all these various other gospels. If this one, for example, is really of historic origin, if it actually does contradict the divinity of Christ, that contradiction would cause it’s rejection from the canon by the Church.
That a document is old or its author was around or knew the apostles is interesting, but does not affect the faith or teaching of the apostolic Church.