The ur-creationist (together with Henry Morris), and co-founder of the Institute for Creation Research, Gish was in the 50s, 60s and 70s the king himself of the creationist debate circus (a throne he left to Kent Hovind who – when you thought it could not deteriorate further into stupidity – yielded it to Ken Ham; at least the creationist talking points aren’t evolving towards intelligence or sanity, but then they were hardly particularly intelligently designed either). The rhetorical technique known as the Gish gallop is the namesake of Duane Gish. It's really a form of snowing, enthusiastically adopted by global warming denialists (the desperate ignorance behind this one is as telling as it is depressing), crackpots and conspiracy theories everywhere.
Duane Gish was indeed a professional debater, and does indeed have a PhD in biochemistry from Berkeley. He has written several books, most famously “Evolution: The Fossils Say No!” from 1978, which has been widely accepted by anti-evolutionists as an authoritative reference for creationist concepts – tells you something about them, doesn’t it? More details can be found here.
Gish has been the peddler of most known creationist arguments (short of Comfort’s banana; that one is in a class of its own), and was the inventor of several of them. Especially famous was his bombardier beetle argument, which Behe later redressed as “irreducible complexity”. In fact, Gish and Morris must be considered something like the inventors of debate-style creationism in the US and the standard set of creationist arguments.
Massimo Pigliucci, who has debated Gish five times, noted that Gish ignores evidence contrary to his religious beliefs – a heartwarmingly tactful statement. A rather creationism-friendly assessment of his work can be found here.
It may be a little less than tactful, but remember that Gish is a guy who claims there is no evidence for evolution. Then this novel came out. It is hard to avoid noticing that cover picture.
Diagnosis: Perhaps the Supreme Crackpot; impervious to evidence, reason and science. Admittedly a good orator, and must be counted as one of the main people behind recent times’ surge of creationism (by moving the debate from scientific evidence to rhetoric). Has done possibly irreparable harm to civilization, although he seems to be retired by now.
Remember this piece of advice, and note what happens when creationist researchers actually start considering evidence rather than talking points. Currently Glenn Morton is writing stuff like this.