The concept of gambling as a form of mental sports refers to the deployment of mental strength in some skill-based games and without physical display of strength or fitness.
Games like Chess, Blackjack, poker and even Sports betting fits in this criterion since professionals employ some good strategies, calculations and even critical analysis before taking the decision on your next line of action. These mental involvements stretch your mind and success in them is a product of strategy, constant practice, psychology and accurate decision under pressure.
Some might argue that Chess might not be considered as a sports, just saying. It's not still recognized by IOC as part of the Olympics games.
Nevertheless, with regards to those, it could be mental game, but if you look at it, just a small portion of gamblers are focusing on this game. And I will say that it's not that famous are compare to most games that we have in casinos.
Again, I'm not against this so called mental sports, but if someone will have to play with it, still involved money and still gambling per se. So we can't really deny that there are still positive and negative effects of it.