You only need to be logical when answering the patient, if he telling me that gambling may make him rich in the future, the perfect answer to this is giving him stats of the people who got rich while gambling compared to all the gamblers who have lost so much and got poor.
First treatment for these patients will be convincing them that gambling irresponsibly is a sickness and can ruin their lives. Knowing that most addicts already knows how bad gambling addiction is, the issue is not about that but how to convince them to not gamble again. Because their problem here is like drug users chasing that dopamine feeling or adrenaline rush, all aware of the bad side effects coming with it.

A rehabilitation clinic will receive mostly patients that are already convinced of how bad gambling is and not the opposite, gamblers who still believe that they can get rich or win one day with much hope they wouldn’t accept to get treated, instead they will find any way to get more money to waste.