There seems to be no mention of it in the core rulebooks. What's up with that?

It's in the supplements. Fly Casual, Suns of Fortune, and... The Jewel of Yavin(? I can't remember for certain about the last one).

I think there's a page in Suns of Fortune.... Basically everyone makes a Cool check (assuming it's against others, not a slot machine), and the results determine winnings. Deception can be rolled if you want to cheat and have a way to do so. It's no Casino Royale tournament rules, by any means, but it gets the job done.

If you wanted to do a tournament, I would recommend elimination. Don't track who has how many chips or credits, just have all the gamblers roll, and eliminate the gambler with the lowest result. Maybe the highest roll gets a boost in the next round, or that could just happen through Advantage. Advantage could also be upgrading others' tests, giving them setbacks, applying strain, etc. If it's the climax of a session, maybe you could be giving out strain, and having side plots amp up the strain, making it feel more tense and like your poker face is about to crack.

tl;dr: Roll Cool and go with your gut.

You can save your money and check out the rules for Sabacc in the free adventure Under a Black Sun.

Or if you just want to get things done quickly, have everyone participating make a competitive Cool or Deception check based on their playing style (analytical or bluffing). The highest roll wins the pot. There is a mention of it in the EotE core book, at least; I don't recall whether it's in the Skills chapter or someplace else.

It depends on if the gambling is the "I want to sit in at the sabac table and play a few hands while the party member who has something real to do does that." or if you are having the sabac game decide if the players get some information, a special gizmo or a ship. Then you might want some mechanisms to make the game work without having to actually spend thousands of real dollars on a bunch of small tablets and the programming to make them into sabac cards. I am assuming that since you ask the situation is the second one and a simple Cool roll isn't what you want.

To role play out the game I'd get out the percentile dice and have each player involved in the game roll them (they could do so in secret with you or in public, your call). Record the result. These are the relative value of their hands. Then roll (in secret) the hands of all NPC players and record the result. Now, if you have a list of the cards in a sabac deck and the relative strength of the hands you can tell each player what they have (again in secret if you wish) based on those relative strengths. Otherwise just fake it. Then everyone who is playing the game rolls their Cool adding 5% for every success, 10% for every advantage and 15% for every inspiration. Now everyone bets (for this go ahead and get chips, those are cheep). Each time someone raises the bet they roll a Deception Check applying the same 5%/10%/15% form. Charm, Coercion and various Knowledge skills can be applied at the GM's discretion following the same form, however those should be contested by the proper ability of the NPC they are trying to influence. Cheating by way of Computers, Skulduggery and whatever else the PC's overactive imaginations come up with is up to the GM, again contesting them with the proper NPC skills and giving bonuses to the final totals. Also the NPCs might cheat, charm or threaten their way through the game as well. Then, because this is sabac and the cards can change face values have everyone roll the tens D10 again and add it in. Whomever has the highest total at the end wins the hand. Then rinse and repeat.

Any rolls for cheating that fail or generate very bad results could mean they get caught and hilarity is bound to ensue. When they inevitably run out of money (or the NPC does, depending on your evil plan) Negotiation checks can determine the values of material goods. If the goal is gathering information that can be worked into the bidding rounds. If your goal is to drop a McGuffin on them it can be dropped into the pot when an NPC runs low on credits. A well planned out game could very well be an entire gaming session.

Enjoy.

You can save your money and check out the rules for Sabacc in the free adventure Under a Black Sun.

We used the sabacc rules for the game I run, and it was quite fun. The scoundrel gambled with all the party's combined credits, after losing several times, on the final round of the game (all or nothing) he won quite a bit of money.

By avoiding entering betting shops and casinos. I also don't go on betting sites. The adverts on the TV are a bit ...

Oh you mean in game, sorry.

It isn't found in the Age of Rebellion book, but there is a side note area in EotE on page 108, if you have that.

A few of my players have suggested using the Galactic Sabacc Deck developed by Joshua Guskin. With replica Galactic Credits for when the group is playing among themselves.

The decks can be found on his Etsy Page https://www.etsy.com/shop/GalacticSabacc

This basically came from the fact the group uses Three-Dragon Ante at different times during DnD and would like to translate that into our SW games.