You cant convince a gambler to stop gambling until he loose all or have his own stop button like pros do. But even pros dont stop for a long time. Never gonna happen, their addiction on a level of a crack heads.
The only way they stop is if they make such decisions.
I know one guy he stopped gambling, he was doing this for many years and never wanted to quit. What made him. Well, one day for 2 years in a row he start loosing non-stop. And he never had money. After that he made decision to stop gambling for good. How long this will last no idea, but he doesn't gamble for 6 month so far.
The only way drug addict stop doing drugs if he get into prison, where every time he ask for the dose, guards will come in and bit him up. And that would last for months. I had a friend like that, he used to steal and do cocaina. One day he try to still a TV and got caught, wasn't his first time. He got into prison for 2 years. Well, guess what guards did to him. I think they did too much, now he hates drugs.
You need to change your image of yourself, your perfection etc.
I think there are gamblers who can still be convinced, those who are just on the brink of addiction. If we can make them realize how much money they are losing then it might hit their head and decide to stop all of a sudden. That's what I did when I stopped smoking, I made a computation of how much I would be spending per day and as sin tax grows every year, it means my bad habits' budget will also grow and I don't like to happen.
That made me realize that I could use all that money for something else like if I want to treat my kids or if I want an item that I could not buy before but since I can save money now, I can.
It will depend on their circumstances and environment too. One gambler might not stop because it's part of his job, or let's say he is living in a gambling den and he is also a worker there. Imagine a daily dose of that, it will be difficult to just let go of it. I have a friend who is near addiction, I can feel it and I can see it, I tried to open it up with him but he got mad about it. This is where it gets rough for friends who literally want to help but they are being blocked. Finding a way to make them open up is also a hard task.