It isn't alleged, it is part of the Nag Hammadi texts. It is as legitimate as an ancient text can be, it just wasn't canonized. That's a mute point, in my opinion. I find the Nag Hammadi texts quite compelling, personally.
What do the Nag Hammadi texts say? What about them do you find compelling? Why do you think their existence hasn't been publicized or advertised to the extent that the texts in the Bible have? I just don't understand why the church was picking and choosing what they put into the Bible and leaving other stuff out.