Both gambling and drugs (alcohol is a drug too) need one small but utterly crippling regulation everywhere in the world. Ban advertising. No advertising of any kind, electronic or physical. No logos, no brands, no specific color patterns if they happen to do gambling or drugs. No advertisement via product placement - so no products on visible market shelves. But otherwise leave them legal.

I'm guessing that as soon as overaggressive advertisement is banned they will immediately lose a majority of fresh marks and victims. During BigTobacco trials those companies showed stats that the overwhelming majority of new smokers are teenagers. As soon as they can't be easily influenced the abuse usage will go down.