I’ve been curious about this subject. Notable games such as The Elephant Hotel and The Plaza have casinos within them, and I’ve seen a very small amount of games be a casino entirely.
So, are these people lucky? Have made alterations that comply with ToS? Or are casinos allowed as long as they don’t involve Robux?
There is nothing in the Roblox ToS that mentions anything about casinos or betting.
It does mention in the Community Guidelines/Rules that you cannot gamble with robux or real life money.
(Section 1 Rules 11)
I hope this helped! 
Edit: Ignore this reply. It’s wrong.
Wait, I was wrong.
In Section II, Rule 1,
It says: " Do not implement any gambling or casino style games that involve real money in any way. This includes Robux, as Robux can be purchased for real money."
Edit: You can create games with casinos that doesn’t involve irl money/robux. For example, if you create a separate in-game currency, that can be used for in-game casinos.
I just thought.
Many countries have outlawed gambling. How would that work then?
Roblox Community Rules explicitly mention gambling involving real money, which includes Robux.
In a nutshell, the players must not be able to purchase the currency that is used for casino using real money, Robux or any other currency that can be bought with Robux.
You should also make the casino games fair for the players, and not skewed in favor of the developer.
Although, even if you make a casino game with all the rules in mind, Roblox might take down the game for review if you mention casino in name and/or description, just to make sure you didn’t break those rules.
Much better explanation then mine. Follow what he said.