Regarding the alleged authorship of John:
From The Interpreters One-Volume Commentary on the Bible Including the Apocrypha with General Articles Copyright 1971 by Abingon Press 15th Printing 1994:
According to Massey H. Shepard, Jr., in his introduction to the Gospel According to John in the section titled:Author. The acceptance of the gospel in the NT canon in the late 2nd to early 3rd century was a seal of acceptance of its authorship by John son of Zebedee, one of the 12 apostles of Jesus. Though contested at that time, this official view held the day without serious challenge until recent times; and is still stoutly defended by many able scholars, Catholic and Protestant. Its strongest support is the testimony of Irenaeus, who claimed to have received the tradition firsthand, when a youth, from Polycarp.
The tradition would perhaps be stronger if it did not claim too much, for in addition to the gospel it places under John’s authorship the three letters and Revelation. Distinguished theologians of the ancient church, Bishop Dionysius of Alexandria and Bishop Eusebius of Caesarea were doubtful that the same hand produced both the gospel and Revelation. They were keen enough to note the differences in these writings both of literary style and of doctrinal viewpoint. They resorted thereafter to a 2-John hypothesis: (a) the apostle, who wrote the gospel and the letters; and (b) a “disciple of the Lord,” who composed Revelation. Support for this thesis was found in a book of Oracles of the Lord by Bishop Papias of Hierapolis, a contemporary of Ignatius and Polycarp, who distinguished 2 Johns: (a) an apostle, one of the 12; and (b) a disciple, who lived in his own times. Papias was conversant with all the “Johannine” writings, though he preferred oral to written traditions. But it is not clear from the surviving fragments of his work to what John he ascribed the books under that name. Many modern scholars reverse the judgment of Dionysius and Eusebius by ascribing Revelation to the apostle – as did Justin Martyr, - and the gospel and the letters to the “disciple.”
The gospel itself has an appendix has an appendix (ch. 21), which includes a colophon (vss. 24-25) ascribing the “witness” of the gospel to the unnamed “beloved disciple” who lay close to Jesus’ breast at the Last Supper (c.f 13:23-25; 20:2; 21:20-24). No reader of the gospel who was not familiar with the Synoptics and Acts would identify the “beloved disciple” with John, or with either of the “sons of Zebedee,” who are mentioned only in the appendix (21:2). But the church in Asia made this identification, as is clear not only from the testimony of Irenaeus but more especially from a letter of bishop Polycrates of Ephesus (ca. 190) preserved by Eusebius. In listing the “great luminaries” who have “fallen asleep” in Asia, Polycrates mentions first Philip the apostle, whom he confuses with Philip the evangelist of Acts, and his daughters and then John, “who leaned on the Lord’s breast, who was a priest, wearing the sacerdotal breastplate, both martyr and teacher. It is notable that he does call not John an apostle, as he does Philip!
The colophon (21:24-25) distinguishes 2 stages in the composition of the book: the “disciple” who bears witness, and “we” who attest to the truth of his testimony. This suggests a posthumous publication by disciples, or an editor, of the eyewitness disciple. Indications of editorial revision have often been noted---e.g. 2:21-22; 4:2 seem obvious, not to speak of the appendix itself. There are abrupt transitions both of the geography and of discourse. Chapter 6 would seem to make more sense if it preceded chapter 5.The dangling summons of 14:31 “Rise, let us go hense,”intrudes in the middle of a long discourse; and the logic of argument and exposition in chapters 7; 8; 10 is curious. There is no manuscript evidence to support any transpositions of the text; nor is there evidence that the gospel ever circulated without the appendix. Nonetheless editorial work seems plausible.
There is a growing consensus that the author – whether “disciple” or “witness” had access to good historical traditions stemming from Palestine, no less than the writers of the Synoptic gospels. His facts, as well as his interpretation must be taken seriously. He knew the geography of Palestine and the customs of the Jews better than Mark, and he may have had Judean associations more immediate than those of the Synoptic writers.
He was undoubtedly a Jew, one whose native tongue was the Aramaic spoken by Jesus. He thinks and writes in a Semitic idiom; and the sayings of Jesus he records, however different in style from those of the Synoptics, betray the same Semitic parallelism of structure. Yet he writes a clear and grammatical Greek. Efforts of some scholars to prove that the gospel was translated from Aramaic have not won general acceptance. His Hellenistic culture has perhaps been exaggerated, but it was not negligible. He was more than match for his theological opponents.
to summarize, it MIGHT be John OR ANOTHER John, but AT LEAST it was a Jew. that is, if there was only 1 author...