Brainless means something stupid or foolish. That is enough to let you know that slots, dices and roulettes are not brainless (stupid or foolish) games.
You gotta take the meaning of the word in context of statement, I meant, "brainless" word here as in playing games that does not require using brain.
You could say that many everyday activities are "brainless," in a way. For example, when I mow the lawn, I do not have to use my brain. Does that mean that I lose a few brain cells every few days for that activity? Thats a dumb thought. The human brain is always working, even when we are sleeping! So, a person who spends a couple of hours a week doing a brainless activity is not going to become dull. However, a person who spends the majority of their waking hours in a passive, consumptive state is probably going to see at least a decline in their cognitive abilities over time.