Rudolf Bultmann's notion of the Gnostic Redeemer-myth has certainly not gone unchallenged. In fact, some scholars have outright taken it to task. See, for instance, the comments of O. Betz in "The Concept of the So-Called 'Divine Man' in Mark's Christology" (Studies in NT and Early Christian Literature, pp. 229-240. Edited by D. Aune)
Martin Hengel also has written extensively about the origins of Christology in his famed work The Son of God, The Origin of Christology and the History of Jewish-Hellenistic Religion (Philadelphia, PA: Fortress, 1976); unfortunately, Hengel's depiction of the relationship between Judaism and Hellenism is also governed by certain presuppositions that seem to adversely affect Hengel's interpretation of the actual Sitz im Leben for first century Palestine. Cf. L. H. Feldman, "Hengel's Judaism and Hellenism in Retrospect." JBL 96 (1977): 371-382. At any rate, I think Bultmann too hastily draws parallels between John's Gospel and Gnosticism where they likely do not exist.