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.
I think I will disagree on this, I really don't know what's up with all the goodie goodie image that gamblers or some not even gamblers in the first place paint gambling as some sort of activity thats they can get good with , for me I think they are probably better and way safer options when it comes to the mind training and I believe gambling is an exact opposite of this all as it will ruin your mind rather than even develop it.