If someone wants to train their mind, 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, it's result will be different for every individual, it can only be used like that and would prove positive result in 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 game it is.
You’ve made a very good point here and if the OP can also view it from your own perspective, he’ll understand that mind games are not better played in gambling spaces but other places where money won’t be involved. As long as that money is involved, you will at a point feel that your mind is stressed out and your mind will actually make you to think about nothing but the money. When it reaches this stage, your focus will make you make many mistakes and from trying to want to correct the mistakes or get back at the market, more mistakes are made and you may find yourself addicted in the end. Different people may observe it differently, but in the long run, an obstruction will come to divert one from the main objective of the whole thing.