Bultmann proposing an early Mandaean Gnostic source for the Gospel of John - with a nice tale about an ex-Mandaean Gnostic convert to Christianity as author really shows the lengths that brilliant scholars go to to defend their own theoretical dogma. As it turns out, those arguing for a Jewish origin against the grain of scholarship at that time were right. With the discovery of the Dead Sea Scrolls we have come to appreciate the imagery of the Gospel of John as fitting the context of Palestinian Judaism.
What was often under attack was the prologue of the Gospel of John. We have Jesus presented as the divine word; we have imagery of creation and a sort of dualism present. Therefore it must be Gnostic?
Regarding the prologue, I would love to quote two great Johannine scholars:
We hope to show below that OT speculation about personified Wisdom and the vocabulary and thought patterns of sectarian Judaism, like the Qumran community, go a long way toward filling in the background of Johannine theological vocabulary and expression. Since these proposed sources of influence are known to have existed, and the existence of Bultmann's proto-Mandean Gnostic source remains dubious, we have every reason to give them preference."In the end, it is unnecessary to look for a background in later Gnoticism (mind the anachronism that would be forced here). Jesus as the logos and creator is an intentional reference to Genesis 1. In John 1:3 we read, "All things were made through him, and without him was not any thing made that was made." The Word through whom all things were made is identifying Jesus as part of the identity of YHWH from Genesis 1. God spoke the world into being through His word.
Raymoned E. Brown, The Gospel According to John I-XII p.56
We do not need to postulate any background to these verses other than Genesis 1 and the tradition of Jewish creation accounts dependent on it that speak of God's Word as his instrument or agent in creation. (Monotheism and Christology in the Gospel of John, Richard Bauckham)
When John uses the term "word" in the opening verses of his prologue, he means simply this: the divine Word that all Jews, on the basis of Genesis, understood to have been active in the creation of all things. (Monotheism and Christology in the Gospel of John, Richard Bauckham
This is typical Second Temple period Jewish thought and exegesis.