Well, how gnostic must a boook be to be called gnostic? There are many books that are more or less inspired by gnostic concepts and there are also books that are inspired by things that are related to gnosticism, like neo-platonism or hermetic magic. Mary Gentle's
Rats & Gargoyles deals with many such concepts, but I'm not sure it's gnostic. The Encyclopedia of Fantasy's entry for "Gnostic Fantasy" mentions Roger Zelazny's Amber series, E. R. Eddison's Zimiamvia trilogy and C. S. Lewis's
That Hideous Strength and
The Last Battle. The article also says that "Franz Kafka and Jorge Luis Borges can be interpreted as gloomy modern Gnostics for whom the fallen world is an endless labyrinth of texts and sophistries, without any exit to trancendence." Other works that might be of interest are H. P. Lovecraft's Cthulhu mythos, which may be inspired by gnosticism, various works by Robert Charles Wilson, for instance
Mysterium, Philip Jose Farmer's World of Tiers series and also Guy Gavriel Kay's "Fionavar Tapestry" has a rather gnostic cosmology. You might want to search Google's Usenet archive (
http://groups.google.com) for "gnostic" in the group rec.arts.sf.written to get some more suggestions.
Last year, I read an interesting book,
The Holy Grail by Malcolm Godwin, about the grail mythology. One of the theories Godwin had was that the gnosticism one one of the central traditions behind the grail myth. It is a pretty good book, the scholarship isn't exactly impeccable and it is rather anti christian in tone, but it brings out tons of nifty ideas from the medieval grail romances that easily would fit into a Wolfe novel.
// Jesper Svedberg