If someone wants to train their mund, they can still go for other mind games that doesn't actually require them to gambling with real money, some people can feel convinced that gambling can train their mind but once they dive in, it becomes a different story entirely. I don't disagree with you that gambling is a mental sport but in the way you described it's result, it can only be used like that and prove positive by some individuals but for many people, you will get an opposite result since they would have a biased thought, not to stretch their mind again but to make money from what ever the game is.