On a mission to figure out a rhyme or reason behind the use of the colloquial “sez” in Gravity’s Rainbow, I rescanned Section Two.
Here’s a quick chronicle from my 1995 Vintage edition copy:
p. 223 the “ID bracelet. Sez KATJE BORGESIUS” [Slothrop’s learning something]
p. 238 “‘It’s Slothrop,’ sez Bloat” [Bloat’s feigning discovery]
p. 239 “‘Shit,’ sez Slothrop” [Slothrop’s in pain]
p. 245 “‘Oink, oink, oink,’ sez Slothrop” [Slothrop’s gleeful]
p. 250 “‘Bad Guy,’ sez Slothrop” [Slothrop’s wishfully thinking; he’s boastful]
p. 264 “‘Hey, Katje’ ‘s all he sez” [after the classic ‘he rapes her but she likes it’ scene; Slothrop’s tentatively probing {not like that}]
p. 266 “‘Come on upstairs,’ sez Slothrop” [Slothrop’s hoping and timidly probing]
p. 271 “‘Your Interested Parties again?’ sez Rollo Groat” [Slothrop’s timidly probing]
p. 288 “Flagnote on the flagnote sez” [major revelation about plastics]
Sez is not the only word the narrator spells phonetically. There are other instances of colloquial spellings. Slothrop says, “whyzat?” on page 287. The Webley Silvernail section on page 272 is all in phonetically written dialect with “dey wuz” kind of theater.
So why the “sez”?
It’s not when someone is being a phony…
It’s not when someone is being dumb…
It’s not when someone is feigning casualness when in danger…
It’s not when someone is on the verge of an important discovery…
It’s used for men and for inanimate objects with writing on them but never about women…
It’s only the narrator who uses “sez.”
So the best I can come up with is it’s an inconsistent narrator tic that serves to remind us of the fundamental unreliability of our narrator? That much is writ large in the 294 minute detailing of Tamara and Perlimpinpin’s debt, the long description of which ends with “Something like that.”
Perhaps the narrator uses “sez” when he’s being particularly intimate with the reader. Clueing us in on something important, peeking in on a private moment, being technically insubordinate to let us see what he’s seen. (I’m assuming the narrator is male. He is visually fixated on genitalia, and I could find pages of clues to convince you, but until I hear a good argument for the narrator being female, I’m going to default to my IPR argument and say we’re being told a story by a man.) At least two-thirds of the above listed “sez”s fit that intimation idea. But two-thirds is not enough for me.
Is it, perhaps, a slip, when the narrator lets down his guard because he’s most engaged in the story? Do we see his true voice rather than his storytelling voice when he’s enrapt with the details of the novel? That fits 100% of the above incidents, presuming the narrator is engrossed in the most significant bits of the book’s prose. So our narrator is pretending to fit in and have clearance to get the gig of telling us the story? That means his true self, the “sez” self, revealed when he’s not paying attention to his persona, is younger, less well educated and connected, and less experienced than he pretends?
I couldn’t find the etymology of “sez” or when it came into fashion. I sense that it was a post-Flapper flippancy that gained ground in the ’40s (as Paul noted) or with Beat writers in the ’50s (as Daryl noted).
In none of the historical slang dictionaries have I found an etymology for “Jackson,” Slothrop’s infrequent pet phrase, either. I have found, though, that Slothrop uses “Jackson” in his internal monologue when he’s seriously terrified. To wit: 221 (octopus), 232 (wardrobe’s a fake), and 287 (with Bounce talking about Shell).
I don’t have the e-version, so I’m sure I’m missing some sezes and some Jacksons. If you come across some, do they help or hurt my theory?
Infinite Zombies 
I feel like “Jackson” may be military slang. A quick search of the Google book version of GR seems to put most instances of it in a military context, or at least in the mouths of army guys. Especially interesting is the one on page 61:
Weisenburger glosses a “ruptured duck” as a screaming eagle pin given to soldiers upon honorable discharge — so another bit of military slang right up next to “Jackson.” I wonder if the apparently brilliant but ill-fated Stonewall Jackson’s name can be found in the provenance of this bit of slang.
Pynchon has a son named Jackson, though he will have been born way after GR was written, I believe.
I believe that it comes from the hipster/jazzbo, “Solid, Jackson” some time in the ’30s or ’40s. I think Barbara Stanwick said something to that effect in _Ball_of_Fire_. The whole phrase in the book as that same feel.
For sez, try this search. Mrs. Quoad is given an attribution with “sez” at one point. Might be interesting to look at these next to instances of says.
I’m sort of leaning toward thinking “sez” is just a little tic, another of those goofball things (like the songs) that Pynchon tosses in when he’s in the mood for it. Could be that he just used it when it felt right to him, without any formal set of reasons for doing it, which of course wouldn’t invalidate its use or prevent an enterprising critic from decoding exactly what sets of contextual circumstances put Pynchon in the mood.
Note also that Pynchon uses “sez” twice in his Introduction to “Slow Learner.”
I didn’t realize until the Pokler section (boy am I a slow learner, speaking of, Marco) that there are several narrators in this text.
That helps a lot. My working thesis, until I have time to researrch what others have found, is that the “sez” narrator is Slothrop’s surveillance tail, just as goofy and earnest and gullible as Slothrop is.