In First and Only, Gaunt discusses with Rawne the defense of their barracks from rival regiment with training shock-poles, and casually tazes a random trooper right next to him as demonstration.
In the opening scene of a short story Of Their Lives in the Ruins of Their Cities, Trooper Caffran watched newly-appointed Ibram Gaunt who was working on what Caffran assumed tactical orders with fascination. Cut to the commissar;
A better tent.
Rawne's remark about his commander, mentioned in Necropolis:
Also doubles as an Awesome moment in Sabbat Martyr: Cuu mouthing off to Gol Kolea as usual, only to then discover Kolea has been healed of his mental incapacitation by Saint Sabbat. After ripping into Cuu for bullying a brain-damaged fellow trooper, Kolea orders Cuu to get out of his sight; Cuu swiftly runs out of the barracks with his tail between his legs, much to the delight of the rest of the regiment.
When Beltayn kills a Gereon occupation officer as he steps inside a shed while searching a farmstead where some of the Ghosts are hiding out.
Really, any time Beltayn says something's awry counts, mostly because it's usually Understatement of the millennium.
In His Last Command, Ezrah doesn't get why Ayatani Zweil is bugging him... so he looks at Gaunt and asks if he can kill the man.
A book or two later, during a brief first-person review of Eszrah's thoughts, he seems to have decided that old Zweil is The Fool, suffered to live because his half-wittedness amuses Gaunt.
This bit about the Eighty-First-First from His Last Command :
Maggs to Mkoll: "You're a piece of work. I've only known you for about twenty-four hours and you get me into this shit."
Mkoll: "Just imagine what I could do if I was really trying."
Even more Hilarious in Hindsight, as Mkoll proceeds to get Maggs into an even more dangerous version of that same situation (outnumbered on a hill with the enemy charging, only this time on a daemon-planet) later on, and Maggs calls him out on it rather than focus on their predicament.
Maggs: Is this what you meant? The shit you could get me in if you really tried?
Mkoll: Shut up, Maggs.
The introduction of the field commander of the Kolstec Fortieth, Forwegg Fofobris, in His Last Command. The entire section is gutbustingly funny:
...Baskevyl and Wilder had taken to referring to Fofobris as Foofoo Frigwig, which was a bad habit, because it was all to easy to slip and call a man by his nickname to his face. The pair of them had once developed the name Jonny Frigging Glareglasses... Wilder had called the man that by accident during a briefing, hed been challenged to an honour duel.
Foofoo Frigwig came on the line. Wilder, is that you, sir?
Wilder suddenly got a fit of the giggles. He remembered a moment in the Post 36 billets, several nights earlier, when Gol Kolea and Ban Daur had introduced the Belladon officer cadre to the mysteries of homebrewed sacra... It was during that little session that Baskevyl had come up with the name Foofoo Frigwig, adding that the foofirst class arsehole was in command of the Kolstuck Foofortieth fighting foofor the Golden fooFrigging Throne.
Ah well, it had been foofrigging funny at the time,...
...Whats the matter with this link, man? It sounds like youre giggling.
...Fooforty minutes. Really, still funny.
During an intense firefight in His Last Command between a reconnaissance team and a surprise enemy assault, Trooper Hwlan gets injured and is lying down while the rest of the team tries to hold off the overwhelming number of Blood Pact swarming up the hill towards them. Then Bonin hears an ominous rumble.
I dont like the sound of that, Bonin said suddenly. Maggs cocked his head. No, me neither, he agreed.
The first stalk-tank lumbered out of the undergrowth, trailing vines and brambles from its spidery legs. Behind it came a second one. The Blood Pact sent up a fierce cheer.
The sound of the cheer roused Hwlan from his stupor. Are we winning? he asked.
Rawne: "That probably sounded better in your head, didn't it?"
Wilder: "Yes. Really, much, much better."
In "Sabbat Martyr" Rawne, Banda and Feygor are arguing whether or not Saint Sabbat is staring at them in particular, Banda says it has to be Rawne because:
"The major's sex on legs, real catnip for us womenfolk"
"Sabbat Martyr" Curth telling Gaunt she'll only do him a favor because he's so good in bed. In front of the whole regiment.
It backfires on her though as the regiment stares at her as if she's serious, making her retort that it was a joke irritatedly. Though that reaction alone is also hilarious, as everyone in earshot (which, as they're getting off their landers, is most of the regiment) immediately believes it. A few slack jaws and a couple dropped kitbags are mentioned.
The joke goes sideways later on when Curth reveals to Gaunt in Salvation's Reach she'd been pining over him for years.
When the events of Blood Pact cause Gaunt to make an unscheduled and incognito visit to the museum of the fallen Tower of the Plutocrat on Balhaut, he finds there are several chapels dedicated to the heroes that died taking it. His reaction on finding on dedicated to Commissar Ibram Gaunt of the Hyrkan 8th is priceless, and all the more funny that neither he nor his party can correct the mistake...
Tour Docent: And this is where the great Commissar Gaunt died while valiantly taking the gate.
Gaunt:(turning to his group) Honestly, I didn't. (cue the sniggering)
There's also some fridge humor. Gaunt has been on Balhaut for garrison duty for two years by that point, and apparently he never bothered to go there and see his past glories. You'd think that some random Tanith trooper would have gone there and told everyone back in the barracks or something...
In Blood Pact, Mabbon (a defector from the main Chaos force the Ghosts have been fighting for years now) and Gaunt have this exchange when the latter is helping the former keep running from both Blood Pact and the Inquisition after an old wound reopens.
Gaunt: Not far now, magir.
Mabbon: You're a good man, Gaunt.
Gaunt: If they take us, please don't say that to anyone. Tell them I'm your sworn enemy.
After the liberation of Vervunhive by the Tanith, Surgeon Curth saw Dorden refusing to rest and over-working himself tending the wounded, so she spiked his drinking water with sleeping drug, telling Corbec not to report the incident to Gaunt. Even Corbec sniggered.
Shortly after Feygor gets an augmentic voice box that makes everything he says sound sarcastic, someone suggests that from now on he should raise a hand to let people know whenever he's actually being sarcastic. Feygor's response:
Feygor: "Oh, that's a good idea." (beat) *Raises his hand*
Dorden's rebuttal of Hark when the latter tries to dissuade him from going to the front.
"I swore," said Dorden, "on the Founding Fields at Tanith Magna, to serve the Imperial Guard and the Tanith First for the rest of my days. Are you, an Imperial fething commissar, really going to stand in the way of me performing that sworn duty? Because if you are, that's one staggering irony."
The Ghost finds themselves a target of ship-boarding with every single communication going haywire. Brostin later suggests the Suicide Kings to link up with the rest, possibly leaving Mabbon they're protecting vulnerable in the process. Right when Mabbon is sitting next to him.
And right after that, when Brostin tells them to go out and kill some shit.
Hark: "Not funny, sergeant."
Mabbon: "Sorta funny."
When Gaunt finally returns to the Ghost's barracks after visiting High Command for debriefing. Bonin notices how the staff convoy is flying pennants, and realize that "some fething lord fething general" is arriving. Commissar Hark starts getting the Ghosts ready for a surprise dress inspection. Several of the Ghost officers prepare to greet this high ranking lord, encouraged by the sight of four elite stormtroopers... only for Gaunt to walk out, ignoring them completely. He's just so annoyed by these events.
Rawne: Sir, what's the big fuss?
Gaunt: Them? They've been assigned. To me.
Kolea: What for?
Gaunt: You know, I've been asking myself that.
Kolea: Theres no one else in the transport? No lord general about to surprise us with an inspection?
Rawne: No one important?
Rawne pulls open Gaunt's coat, revealing his insignia as a Lord Militant Commander, and Kolea promptly demands to know if it's real or not. After Gaunt's bodyguards nearly attack Rawne for opening the coat and Gaunt orders them to face the wall, Kolea then asks Gaunt if he's fething kidding, because he still can't believe this is actually happening after so long.
Rawne: What the feth is this?
Gaunt: Well, Im going to tell you about that...
Kolea: Is that real?
Rawne: Youre a fething militant commander?
Gaunt: I fething am, Eli
Kolea: Are you... fething kidding?
Gaunt's retinues are offended at the Ghosts' friendliness and usage of crass language. Gaunt tells them to literally face a wall.