Well, the big thing is beyond the rules and is how you play the game.
I toss out all modern ideas, like balance, fairness, "the gentleman agreement" and "the way everyone else does things". Or, really, just play like it's 2E and or 80s/90s.
Anytime Random Meaningless Character Death is a big one to make a game gritty.
Very Low or No Information: The Players are very much in the dark about nearly everything in the game. Specifically the DM does not treat them like "readers/watchers" and tell them things about the game that the characters will never know.....but makes the players "feel better" to know.
Unreliable Information: Even the small trickle of information the players get is very often mostly untrue or worse. Nothing in the game is "page 11 says so, so it must be so".
Out Side the Box: This is another big one as most players like to put everything in little rule boxes and say "this is this, so say the rules". Go outside that box....and it's pure grit.
Resource management another big one.....it's very gritty. You can't be a super hero and just blast away with harsh limits.
Lingering effects: Things that effect the character very often can not be just 'fixed' by a low level spell. A lot of things take much more then that. Often a character will have to live with something, like a curse, for years.
Damaged and Lost Items: Lots of grit here.
Foes using tactics: More grit