I thought PbtA might not be the good fit for gritty gaming.
But Zombie world mechanics makes death an unexpected threat.
And the bite deck even induce a sense of fear.
I tought i’d only cherish that feeling on Osr games since Pbta usually supports character development.
Tell me fine gauntleteers do you know other PbtA games with a similar intent ? 
I think some of this depends on what you mean by gritty? Like, if it just means character death is common, it’s probably true that most PbtA games aren’t gritty in that sense. (Though there’s no inherent reason they can’t be. As you Zombie World is one example, I’d say Freebooters on the Frontier also allows for death to come often). Grittiness can mean other things though. Apocalypse World can be pretty gritty, in the sense of a game of violence and resource engagement where the world is out to get you. I’ve played gritty games of Dungeon World. Night Witches gets gritty, in terms of danger, getting by on the skin of your teeth (and that’s another game where character death can happen somewhat frequently). Urban Shadows and SCUP can both have a gritty tone, in terms of a world that is against you, where constant negotiation is important just to get by. Gritty doesn’t have to mean “a character can easily meet a violent death.”
Independently from what your personal idea of grittyness involves - the question is how
that feeling is evoked. If we are talking about grittyness which is induced by mechanics it may be hard to look at PbtA and seeing the obvious mechanics as a factor.
If you - on the other hand like in more traditional games - have many stats and more or less an extensive book-keeping about the current state of the characters and the world you could leverage these to tighten the thumb screws on the characters (players). If you e.g. use time and resource management rules you could leverage this mechanic to make the game feeling more gritty: Think of being on the 9th level of a dungeon and your last arrows are shot, your swords are broken and your last torch goes out - just now.
Or think of the accurate bookkeeping of time which leads to calulations about the lasting effect of spells and the time slot needed to do x.
One concrete mechanic from older editions of DnD which obviously induces feelings of grittyness is level drain: Your are thrown back to a previous state with less power in a currently extreme dangerous environment.
I am only acquaint with some PbtA systems and there resource management is abstracted away - of course if your even abstract resources are gone you start thinking. The same goes for the classical HP management. But as far as I know something like level drain is nowhere established.
If you take DnD and Dungeon World there are rules for healing and recovery which could be leveraged to mechanically increase grittyness. Examples given in the DMG could be translated to Dungeon World.
But from my understanding of the PbtA philosophy the best mechanical screws for grittyness in these games are GM (especially hard) moves as a result from reaching a turning point towards the unexpected in the story (mechanically as a result of rolling a 10-) and snowballing (where one bad consequence could be only the first in a chain of bad things to happen). In a gritty environment loosing one characters complete food rations when the knapsack goes down the crevice may actually be a soft move because there are much more harsh consequences (hard moves) to be afraid of.
Another lever to pull for more grittyness is the clock mechanics from Apocalypse World which is used (with some iterations and modifications) in some PbtA games: Having these clocks in front of the players may be a mechanical way of visualizing pressure on the players which may have a comparable effect to the stats of trad games.
If you think about the roots of DnD and in consequence OSR it is obvious why mechanics played a big role in inducing grittyness in these kind of games: DnD was a game between different players - one being the GM and the others playing characters. To let the players (playing the characters) feel some pressure you could fumble around with their stats. Whereas PbtA is more about the characters and the story you have less mechanical levers to pull but build a gritty atmosphere through narration.
tl;dr
PbtA is as gritty as you make it.
I personally think Apocalypse World, the original PbtA game, is very gritty.
For OSR-style PbtA I’d recommend Freebooters on the Frontier 2e, currently in playtest.
Thanks a lot gauntleteers I feel enlightened
The more I think about this, it seems the more clocks in PbtA AND the harder the move/ heavier the consequences, the grittier the game feel. It’s the feeling that something is going to get you if you don’t progress the “story”.
I played the D&D 1e version of Castle Strahd this weekend. What made it feel gritty was the the fact that every time we stopped for rest, we got beat up worse. If we stopped long enough, Strahd literally appeared to harrass us. The other bit, was that the vampires literally sucked a level or 2 away with each bite.
In modern game design, this was threat clocks and heavy consequences for hard moves (failing saving throws) but done using old school methods.
In a different example, even though I have never played it, based on these mechanics and what I’ve seen in actual plays on YouTube, the grittiest PbtA may be The Sprawl. The pressure through clocks is intense and the consequences of failures is brutal. Although not high fantasy, this one seems the grittiest to me. Perhaps too gritty as PC failure seems inevitable.
I wouldn’t say ‘grit’ is equivalent to ‘pressure’, personally.
I mean, Night Witches is the single grittiest RPG experience I’ve ever had, so yeah.
I think Zombie World is fascinating in this context, because it uses a PbtA core to get at some very OSR-feeling game aspects:
rapid, randomized character generation
death very much on the table for PCs
A lot of latitude for players to decide how they’ll approach the challenging world they face (you can totally play a monstrous tyrant or a peaceful diplomat)
So I concur with others in this thread that PbtA can feel gritty. But Zombie World makes some choices that depart from conventional PbtA (dispensing with playbooks for characters made of multiple random cards) and along the way captures some of what people cite as distinctives of OSR.