Quote from BoboBoombang>>

Can anyone help me here? What do I roll for a ‘tinkerer’s tools check’ to fix my gun?

That's correct, here's the relevant section of the rules on D&D Beyond (under Gunsmith):

Misfire. Whenever you make an attack roll with a firearm, and the dice roll is equal to or lower than the weapon’s Misfire score, the weapon misfires. The attack misses, and the weapon cannot be used again until you spend an action to try and repair it. To repair your firearm, you must make a successful Tinker’s Tools check (DC equal to 8 + misfire score). If your check fails, the weapon is broken and must be mended out of combat at a quarter of the cost of the firearm. Creatures who use a firearm without being proficient increase the weapon’s misfire score by 1.

So a more reliable firearm with Misfire (1) only has a DC of 9, Misfire (2) is DC of 10 etc., so they're pretty easy checks to pass, the cost is really losing your action to do it, you generally just want to make sure you have a spare weapon handy.

My problem is that I don’t know what to roll. It just says ‘tinker’s tool check’, I can’t find out what that is. I’ve tried looking in the Tools rules, tried looking at Tinker’s Tools themselves, nothin’.

Quote from BoboBoombang >>

My problem is that I don’t know what to roll. It just says ‘tinker’s tool check’, I can’t find out what that is. I’ve tried looking in the Tools rules, tried looking at Tinker’s Tools themselves, nothin’.

Ah, yeah the tools rules in 5e are a bit vague; your DM picks an ability score that's relevant to what you're doing.

Personally for repair/unjamming, I'd call for a Dexterity (tinker's tools) check, so the roll would be 1d20 + your Dexterity modifier + your proficiency bonus (for tinker's tools proficiency). Whereas for crafting I might use either that or Intelligence (tinker's tools) for crafting or such.

It's up to your DM really but Dexterity is the most obvious unless you're working with a homebrew cannon that might be better as Strength.

Damn, I suppose that’s fair enough. Ima ask my dm to make it either a dex or wisdom check, cause wisdom is the score Gunslinger uses for some reason.

Thank you.

I'm a bit confused as to why grit points are determined by Wisdom when nothing else about fighters relate to Wisdom and it's one of the most common dump stats for fighters (after intelligence).

Quote from RealShiningBike_II >>

I'm a bit confused as to why grit points are determined by Wisdom when nothing else about fighters relate to Wisdom and it's one of the most common dump stats for fighters (after intelligence).

 Its not exactly unusual for a sub-class to use a secondary stat to power sub-class abilities. The Eldritch Knight and Psi-Warrior sub-classes use Intelligence after all.

My guess is that its partly flavour - Wisdom representing determination and willpower - and partly balance: You may not want a sub-class to require too many ability scores, which would make it hard to play effectively, but its also important to avoid "One Stat to Rule them All"!

So is this subclass just not avaliable for 2024 Fighter?  I thought 2024 was backwards compatible?  

Looks like only a handful of the more recent fighter subclasses are available for the character builder.  But otherwise if you don't mind doing things manually it shouldn't be a problem.  Wonder what it would  take to homebrew it for 2024.

You fell for the bait and switch tactic WOTC employed.