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.
You cannot play these games without exerting effort with your mind and body, so it's considered a mental sport, and every mental sport can become a form of gambling if casinos decide to include it in their platform and offer it to gamblers, even if the game is of a physical type, there is also an exertion of the mind, so every sports is considered a mental sports and it need discipline to become the top of this game.