There isn't much grandchildren can do anyway regards the gambling activity of their grandmothers. An elder is an adult responsible for his own acts, have lived much longer than grandchildren, therefore there is a natural belief of being wiser and more experienced, besides having their own source of income. Why would they listen to orders coming from children? If you put yourself on their shoes you will understand it. We don't like to be ordered by younger people than us and people who we consider to be less experienced, and it seems as we get older, this feeling just gets stronger.
I believe the only thing grandchildren can do, which may work, is to appeal emotionally to their grandmothers that they stop. This kind of approach can sensibilize the elders, although it's not likely to have long term impacts...
But if gambling is detrimental to their health, are we going to just keep quiet watching the behavior of our parents who are gambling and risking their money to make a profit? Indeed, some parents don't really think about what their children say and seem indifferent because they feel they are more experienced and wiser, they feel He was right with his actions so he continued gambling even though he had to spend a lot of money. Actually, a child doesn't have a problem if their parents gamble as long as it's for fun and not too much. But a child think about their parents' health condition and worry that it will cause their blood pressure to rise because the elderly are more susceptible to congenital diseases like that.
Yes, the only way is to persuade him, not scold him or tell him to stop because parents don't like being told to, with a good approach and gentle words, I'm sure parents can understand the words of their children who care about them, especially caring about their health.
In my opinion, if they keep quiet about it, it's very wrong, because basically we don't want our parents to experience loss of health, because in my opinion, with those who may be old, it should be time for them to enjoy their old age well, and enjoy their old age by gambling. I don't think that's a good thing, even if they have a lot of money. and worrying about their parents' health is normal, because if there are children who don't worry about their parents, I think that child is a child who is not filial to their parents, because they just ignore what they are doing and it is detrimental to their health.
I agree with what you say, however we must be able to lead our parents to good things, also by treating them well because we should be able to take care of our parents so if they do something that is detrimental to their health, it must be prevented because it is also for their own good.