The ROBLOX Community Rules state:https://en.help.roblox.com/hc/en-us/articles/203313410-Roblox-Community-Rules
To answer your question you can make a gambling game but its in a grey area. I would not recommend you make one as its encouraging kids to gamble at a young age, even if they can’t do it with real money.
The closest you could get to gambling is what they do in The Plaza where they give you a currency where you can gain more currency from winning slot machines etc.
Honestly theres no real way to monetise it. Selling the currency could be considered as gambling with ROBUX. Perhaps you could add a VIP room and tools.
As I have said above though, it would be much safer and much more professional of you to make a game that doesn’t encourage gambling. However it is possible to do provided there is absolutely no way they are spending any real money on the gambling itself.