OP are you going to involve the government of your country about this or just you? I think it's not safe to start such business of lending gamblers money because
1. Most gamblers won't pay back what they own when the amount is too much, and if you threaten them too much you won't be safe either, how many stories of people killing people to avoid paying what they owe? There are many stories like this, and been just you doing this you can lose your life.
2. If something happens that involved the law and authority you will be at fault and they can charge you because you are not registered as a lender, there are standard lending business in the world today, so make sure you get a license and think about good security for yourself.
I don't have Interest in such business because I have seen lenders going through the stress of looking for debtors that refuse to pay the they owe back, some will run off leaving one country for another and you will have to put them on manhunt to get them arrested when they are in the public, if you think you can handle it you are free to do so, but be prepared, gamblers know how to owe a lot of money, most of the ate living a miserable life so they don't care.
That's right, people who are addicted to gambling will do anything, even if it's high risk for them, but it's because of their gambling addiction that drives them to commit crimes that can even involve the law in their country. And I've also heard the story you mentioned, where someone collected a debt from someone who owed him, but the person (who owed the debt) threatened the person who collected the debt. Of course the ones who are at fault are those who borrowed the money, they dare to borrow money to gamble because of the addictive desire to do so they take out loans to gamble again, and it is true to what you said, they will find it difficult to repay their debts, even when they have money, they will be more concerned with gambling because they think gambling can multiply money.
But with the losses they always get, they should realize that they have lost a lot of money because they always choose to double it by gambling. And what I fear is that they will commit crimes that will harm themselves and others who do not know anything. It's not funny to hear people who don't know anything but are influenced by someone who is addicted to gambling. They should be aware of the harmful effects of gambling addiction that will bring big problems in the future, There is nothing wrong with gambling, but with a note that it can control itself well so that it can avoid severe addiction.