Knuckle-knives? Knuckle-knives?!
To quote Naberius Tern.
What are "knuckle-knives", exactly? There are several options, all of which are valid, and whatever anyone imagines themselves, is right! I, however, am the kind of reader who... well, long story short, I hyperfocused and at long last came up with my own, at least halfway decent, idea.
Visual clues are the Tommy Arnold cover. That one indicates a glove with 2/3 of the fingers exposed, and having three <10 cm blades fastened to them. Gideon also wears a bracer. It looks to me like it's separate from the glove.
The descriptions are sparse, but some are informative. Every costume note for every character can be found here, by thinkomancer. I picked some of them:
Page 62 of GtN: "Knuckles: they were even less ornate, being obsidian and steel set in thick and heavy bands. There were three black blades on the back of the gauntlet, rigidly fixed in place." 133: "ebon cuff of the knuckle-knife" 178: "grease-black glass" 134: "peeled the knuckle-knife off her hand and latched it onto her belt" 121: "unbuckling the knuckle-knives from her wrist, easing her fingers out of the grips" 159: "slipping the knuckles of her left hand through the obsidian bands"
Things that are clear: Worn on the left hand, made from obsidian (volcanic glass) and steel, fixed in place, latched to the belt when not worn. Black.
Confusing: Set in thick and heavy bands. On a gauntlet. Knife vs knives. Buckled onto wrist. Have/has a cuff. Grips to put fingers in.
Usage: Gideon blocks with them. This indicates that there's something covering her hand, or at least the back of it, the fingers, and partway up the arm, because otherwise, ouch. When it comes to fencing, as far as I have found, having gloves that protect also your lower arms or at least wrists, are a necessary thing.
I did some proper time-consuming image searching, and there are a few options that come up if you use variations on the theme of "knuckle knives":
First, a trench knife. Military use, several different variations, think a knife, with a grip on which there's brass knuckles. Or brass knuckles with a knife sticking out. The blade would point "upward", like you hold an ordinary kitchen knife. Only a detail or two fits the descriptions.
Second, a "Robins punch dagger", from the Imperial War Museum. Held like a pistol grip, except there's no trigger, no gun, just a knife pointing straight forward, and a metal band that reaches around your fingers. Fits vaguely with one or two details of the description.
Third, a Karate Mart "knuckle knives", uh, contraption. Brass knuckles with three knives that stick out between the fingers. Looks pretty X-Men-Wolverine-esque. But nothing to unbuckle, no gauntlet etc. There are several variants of this, but the principle is the same. The "Wolverine claws" of the same website are variations on the same theme.
Fourth, a "tekko kagi", or one of the items that gets translated as "ninja claws" (this picture from Ninjadojoandstore). Has the bands, possible to fasten, somewhat knifey, probably possible to use for blocking and smashing things out of the way. Kind of protects the hand, too.
Fifth, "hand claws" from Karate mart. These come in a few styles and looks. None with obsidian knives, of course, and I'm not sure I'd use obsidian for blocking, it seems risky for their edge (you chip obsidian for sharpness) and structural integrity (because it is after all volcanic glass, for all that it's not unduly brittle). But... they fit the description best.
If you've played "Dynasty Warriors" you'll recognise these last, and the ~vision of elegance~ that is Zhang He, who uses them as his weapons, one on each hand. For all that they are... almost barf-inducingly pretentious, fantastical and mall ninja tacticool, I think they are what comes closest to the knuckle knives as described in "Gideon the Ninth".
Then again... "set on a gauntlet"? That's "big, long, armoured glove" to me. So are the gloves separate from the rest of the construction or not?
Ultimately, I personally decided to set the blades onto a "band", as best I could with my limited skill set and materials, and put that on a gauntlet-looking glove.
Having spent time on and off trying to figure this puzzle out, I might just have to remake this part of the cosplay. Because I'm Like That.
ETA I want to add: Everyone's and anyone's ideas, images, headcanons, however one imagines the knuckle knives, is valid, and I am in no way trying to say I am an authority and objectively correct. Because I'm not. I just hyperfocused a bit!
Tell me your thoughts on this! Share it around so I can get more feedback and discussion on the matter, I find it a very interesting subject.
