Gurney is one of Duke Leto's chief officers. He's also Paul's weapons teacher, a baliset player, and a warrior famous for his skill. In short, he's awesome plus a bag of chips. Gurney represents the archetypal holy or religious knight in Dune. But before you write him off as Dune's surrogate Knight Templar, we'd better explore him further.
Everybody knows about your typical knight. He's courageous, chivalrous, and loyal, and he's got romantic tendencies to rival the giddiest of school girls. The concept of the noble knight is so old that writers have been parodying it since the 17th century (source). All the same, Gurney Halleck is the ye olde knight character through and through.
He courageously stands by both Duke Leto and Paul during their battles against the Harkonnens. He is fiercely loyal. When he learns Paul is alive, Gurney proclaims, "My lord, I've never left your service. You're the only one left to me…" (43.108). Gurney was planning to get revenge for Duke Leto even before he learned of Paul's survival, anyway. As for the romantic streak, well, just check out any of his songs for an example (44.176). But what about that holy business…?
We say Gurney is a holy knight, but let's dispel some misconceptions here. Gurney is not a religious knight like the Knights Templar, and does not crusade for his religious beliefs. He's not even Dune's equivalent of a mage knight, rolling those D20s to pull off healing spells or magic missile attacks.
No, Gurney is just a knight well-versed in religion and religious texts. Go on and head over to our "Shout-Outs" page and take a look at all those Biblical quotes. Half of those belong to Gurney. The man knows the Bible and the Quran—or, in the Dune universe, the Orange Catholic Bible (more on that in our "Symbols, Allusions, Imagery" section).
Now, this makes Gurney an important character in understanding religion as a theme in Dune. This novel has many instances of characters citing religion as their reason to perform immoral acts—everything from killing to subterfuge, from war to eugenics. With Gurney, we see a very different use for religion. For him, it's for knowledge and wisdom. Gurney doesn't join Paul's crusade because he believes in Muad'Dib. He does it because he loves Paul and Duke Leto. It's no coincidence that he is the first to rebuke both Duke Leto and Paul's poor decisions—with quotes from Exodus (12.118) and Samuel (48.27), respectively.
And that's why Gurney is a holy knight in both the traditional and not-so-traditional sense.