Sorry I have been out one day.
About the relationship of GJohn with Gnosticism, the main flaw of Koester's analysis IMO is actually taking later clear-cut Gnostic Christian works as the Gnostic paradigm for reconstructing the content of GJohn's source and then evaluating how GJohn supposedly departs from such a source. The later Gnostic works such as the Nag Hammadi Apocalypses of James obviously react to the previous impact of some form of "Great Church" soteriology, which is hardly the case of any supposed source for GJohn. This (Koester?s) model, in turn, tends (still IMO) to obscure the very movement of GJohn?s logic, or initiatic pedagogy, which doesn?t bring in proto-orthodox Christian thinking (involving futuristic eschatology, for example) as the ultimate word to ?correct? any protognostic thought, but to the contrary uses it as a penultimate word, i.e. as the very starting point (common, popular or outward understanding) from which GJohn leads the reader into a ?higher? understanding.
Examples:
Common understanding, John 11:23f: Jesus said to her, "Your brother will rise again." Martha said to him, "I know that he will rise again in the resurrection on the last day."
Specifically johannine understanding, John 11:25f: Jesus said to her, "I am the resurrection and the life. Those who believe in me, even though they die, will live, and everyone who lives and believes in me will never die.
Common understanding, John 14:3: In my Father's house there are many dwelling places. If it were not so, would I have told you that I go to prepare a place for you? And if I go and prepare a place for you, I will come again and will take you to myself, so that where I am, there you may be also.
Specifically Johannine understanding, John 14:23: Jesus answered him, "Those who love me will keep my word, and my Father will love them, and we will come to them and make our home with them.?
This pattern doesn?t exclude even later additions meant as ?ultimate words? to give the whole Johannine system an ?orthodox? ring, such as John 5:28f (unnecessary to expose the Johannine understanding which was already given in v. 21-25), or the recurring phrase ?I will raise X up in the last day? in chapter 6. The characteristic of those is that they appear as unneeded additions, as the specific Johannine logic stands perfectly (and even better) without them. In my view, one has to distinguish between the two kinds of apparently ?orthodox? references.
On the Petrine issue, I tend to see it as a very late fabrication, building on ?Cephas? tradition and many others. The articles by Ernst Barnikol and Frank McGuire on http://www.depts.drew.edu/jhc/ are interesting on this subject. But we may come back to this in the next thread.