Boy, it's been a while hasn't it? Let's get into it!
Gambits have been a mainstay of my game from the second they were introduced. Any exciting combat stuff that's not straight "roll hit, roll damage" is covered by either Gambits (roll twice, if both hit you do what you want) or a Wrestle (roll off against enemy, on success do a wrestling move).
My traditional Gambit rule, based on the OG bossman Last Gasp, was essentially "declare a stunt, roll to hit twice, on double success it happens, on double fail it happens to you, otherwise we talk" situation.
The gameplay downside was that it was AC that defined whether you would successfully gambit.
A regular 1HD dude in 18AC Plate resists gambits better than a 1000000HD monster with unarmoured 12AC, that sort of thing.
It also meant that stacking to-hit bonuses would make any gambit a practical certainty, which isn't so good when Gambits can be straight up "I kill all the baddies in one move"!
The best thing about Gambits from my DM perspective is that they're always player-incited chaos, and they're always player-incited cool shit. If someone wants to Gambit they're intending to do something real cool and inviting real consequences.
So from a practical mechanical perspective, the key change is that the new formulation is based on HD instead of AC.
Weaker monsters are easier to stunt on, stronger monsters at the very least need to be taken down a peg before you break their legs.
More excitingly, as the result of a weeb fever dream, there's some extra spicy stuff to add even more chaos into the already chaotic Gambit.
If you want to do something extra fancy, it's a Gambit!
Roll twice against the enemy's Gambit AC. You add your Base Attack Bonus + an appropriate Stat Bonus.
Gambit AC is 10+HD. If the enemy is under half health, their Gambit AC is 10+half HD.
If both miss, it's an ironic reversal! The Gambit happens to you.
If you roll a natural 20 on either die, it's a team-up attack!
Resolve the Gambit, then another character you choose can join the Gambit! This doesn't even use up their turn.
They declare a new Gambit involving the original target and roll with your original modifiers, plus their BAB plus an appropriate stat bonus.
You successfully trip the guard, and your friend says "I'm in! I want to steal her spear and chuck it at her boss".
The DM says that sounds like a Dexterity Gambit, so they roll and add their BAB + Dex mod to your original modifiers. Both hit!
You throw the guard to the floor as your friend catches her spear, and throws it like a javelin at the guard captain!
If you roll two Nat 20s on a Gambit, it's an All-Out Attack!
Resolve the Gambit, then everybody in your party (in any order, including you) gets a free auto-hit attack on the enemy of their choice! They'll never see it coming!
Movement is allowed during an All-Out Attack so long as you can justify it with a suitably cool/grandiose/ludicrous team-up combo move in the fiction.
If you roll a natural 1 on either die, the enemy strikes back!
Before your Gambit resolves, the enemy gets to make their own Gambit.
Your Gambit AC is 10+Level.
Enemies don't usually have stat modifiers, so they only get to add their BAB.
If you roll two Nat 1s on a Gambit, you're swarmed!
Resolve the Gambit, then any enemy that could conceivably target you gets a free auto-hit attack on you with whatever they've got to hand!
Movement is allowed during an All-Out Revenge, so long as the DM can justify it with a suitably cinematic/tragic/bathetic scene in the fiction.
This is probably obvious, but if one of your players rolls a Reversal you should make sure the enemy's Gambit is on par with what your player tried to do.
So if they tried something fairly harmless like trying to trip the enemy, maybe the enemy tries for a disarm or throws the character at someone else to knock them both down.
If they went for something brutal like trying to decapitate the enemy, maybe the enemy tries to chop a leg off or permanently blind the character.
Remember that your monsters are much more replaceable than PCs, so cut them at least a little slack!
If a player wants their Gambit to affect multiple foes, just add enemy HD together to find the target number.
Like three 1HD mooks would have Gambit AC 13.
Two 6HD bears would have Gambit AC 22.
Ignore the half health thing with multiple foes, too much effort!
Example: You declare "I decapitate the bears!" against two 6HD grizzly bears, and you roll a natural 1 on one of the dice! The DM says "lol ok they're going to try to rip your arms off".
They've got +6 to hit each, so their combined bonus in their Gambit is +12 vs your Gambit AC of 10+Level.
These rules are intentionally written so that Gambits can, dice-willing, devolve into absolute chaos with a Reversal conceivably triggering 1 More triggering another Reversal and so on.
That's why I was specific about when a Gambit Special resolves! Reversal triggers before 1 More, which is especially important in the rare case of rolling a Nat 1 and a Nat 20 simultaneously.
Everything always resolves, even if some ridiculous Reversal makes the original Gambit completely implausible, so have fun working it out in the fiction!
For the probability wonks, every Gambit roll has approximately 20% chance to trigger a Gambit Special.
This contrasted with the 10% chance of crit/fumble on a normal attack roll.