Is there any proof outside the Bible for the historicity of John of the Gospel?

There is no extra-biblical proof of the historicity of the disciple John. Scholars say that the Gospel According to St John was not attributed to the disciple John until the middle of the second century. Their reasoning was that while the Gospel never mentioned John, it did mention a "disciple whom Jesus loved", a figure of speech they felt to be explained by the extreme modesty of the author, who must therefore be John.

John the Baptist is mentioned outside The Bible, by Josephus. This John is likely to have been a real person, but it is also possible that Josephus only learnt of him from Christian sources.