For any version of Mongoose or Cepheus:
Poker: Gambler + Int. Possibly a preceding task chain from Deception + Soc if you want to drill down.
Dice: Gambler + Dex. Optional task chain from Stealth + Dex if you really want to risk getting your legs broken.
Darts: Gambler + Dex (+Athletics - Dex), and probably a preceding task chain from Carouse + End.
Chess: Gambler + Edu (assuming speed play and betting here).
Mah Jong: Gambler + Int
and + Luck (if used), or else roll 1d6, 1-3 is +1 to 3, 4-6 is -1 to 3.
Betting on horses: Gambler no stat, preceding task chain from Streetwise +
or minus Soc. (Normies draw the short straw.)
Any kind of high dollar lottery: you're not going to win with a 2d6 roll. Unless you outright rig the game, but then you're into a full session or multi-session heist adventure more than just a skill check.
A side note, any version of Mongoose benefits from using the Task Chain table from the 2022 version of the core rulebook. It's the one that's most worth taking your chances on. Still some chance of a negative, but you're not punished for trying quite as much.
[Player wants to cheat for a plus.]
I assumed you were cheating all along. Just a normal level of card counting, dice control, or whatever else can be assumed for higher skill ranks rather than separated out.
But more elaborate schemes I would usually represent as a task chain.
What actually do you get for playing and winning?
Classic and Mongoose both have answers to this for narrow circumstances, but I found them a little unsatisfying, and potentially too easy to game if you allow multiple favorable bets rather than taking them as for a session of play.
What I've noodled around with is: Set an ante, or a stakes number, for an evening or session of play. So it'll come down to one roll - if you do have an elaborate ploy to make, so much the better, but figure it out and declare it. You may win or lose more than the stakes, so this
should be a fraction of your purse! Roll opposed Gambling checks, loser pays the winner the difference times the stakes. A weak point is deciding what to do with multiple notable players. You could do winner take all, but some games might benefit from pairing highest to lowest, next highest to next lowest, etc. With an odd player maybe compare their roll to the average, and take a share of winnings if higher or pay out if lower... (I feel I could run it faster than I can describe it.)