d20 is designed, in a meta-game sense, for the PCs to end up as deific beings with manga-comic super abilities. That is great fun except when you don’t want those beings to be possible. One might say that d20 is the wrong system however I say it’s an okay system that is like learning blues on guitar. It lets you jam with anyone.
Gritty is just code-word for “dangerous”, or the possibility that you can die quickly in combat: kind of like real life. So here’s some options:
1) Hit points max out as Str + Con + Base Fort Save
2) Critical strikes automatically inflict maximum damage
3) Death starts at –1 HP.
4) Heal spells aren’t instantaneous – they’re overnight; thereby forcing them to be combined with rest.
5) Any critical strike (regardless of if you use 2 above) requires a Fort save of DC 10 + damage inflicted, or the target is stunned for 1 round.
And one can go on.