Gambling can be Deception vs Insight, or Sleight of Hand vs Perception... or raw INT, or Strength (if it's arm wrestling) or Intimidate or-
This comic follows on from the Previous comic which might help provide context.
Why would Konsi play with such a handicap?
Growing up as a street urchin, she never really had access to lots of clothes. When not wearing armour, or dressing up for an event, she’s still most comfortable wearing just a simple robe.
She’s trying to practice keeping her cool under pressure, and that’s not effective if you have a lot of safety.
Oh man, I loved Dusa's interactions. It was nice that being close to Dusa wasn't framed as being "just" friends, because that would implicitly put friendship as less important than romantic and/or sexual relationships. Like, friendship isn't a consolation prize, nor is it "rolling to seduce" and failing, but so many games and other media depict it that way
Gambling would be rolling a d100 with no modifiers and your prize is one of several things I, the DM, have written down on a table. Most of them would be nothing to simulate real gambling.
Video game RPGs often have little minigames for things like this, with your stats helping you manipulate the game in some way. KCD, for example, doesn't have any stats which help you, but you can acquire cheat dice to improve your odds.
It could be fun to play a session like this at the table. Actually play a game of poker or blackjack or something against NPCs, and let players make rolls to help. Some thoughts (using PF2 skill actions):
Palm an Object (thievery): swap your given card for one you've hidden on your body (requires establishing ahead of time that you've got cards hidden)
Lie (deception): convince your opponent to bet big when you have a strong hand, or to fold when your don't (DC scales with how good their hand is)
Create Forgery (society): if they use some unusual or non-standard cards that you couldn't just buy, use this to create your own set for use with Palm an Object. If you're bringing your own full deck you can also subtly mark the back of cards so you know what they are.
Perception: spot the card that has been dealt
Perception: read an opponent to see if they're bluffing
I might also allow some sort of intelligence check to do things like knowing your odds of winning, or to automate blackjack card counting, especially if the characters would be better at that than the players are.
We basically run a system where we deal texas hold'em, and if you succeed at a relevant skill check, you get handed an extra card to use. It's not quite the same outcome for skill use as it would be, but it's a reasonable method to "skew the odds" in your favour.