The Gospel of John, like the other Gospels, is a a complete forgery. No one knows who wrote it and it's very late authorship testifies that the book was not the work of the original Apostle, John. To call it or any of the gospel books the "Gospel according to whomever" is a complete lie, even if tradition created the title, it's still a lie.

Historically, yes, the book provides some detail not found in the other gospels; that doesn't make the book truthful of the life of Jesus nor what REAL period Christians believed. It's the work of, as you allude to, a Johannine community who were not really followers of the true apostle, but of some Pagan (ie. Gentile) tradition of John that was more likely Pauline than it was Johannine.

For the record, Paul was not an apostle of Jesus and was in fact derided and opposed by the true apostles, which also makes the books of Peter outright forgeries as well.

With so much forgery and lies going on within the Bible, how does anyone even call the Bible or the NT "holy"?!?