Disease can be passed to another person or get spread and addiction doesn't do that. It could be categorized as a mental problem that range from mild to a very serious addiction on gambling.
Yeah and mental problem is a disease. So gambling addiction is disease. Well maybe you do not think of mental problems as disease, but I do. Gambling addiction must be cured, if it is extreme and the gambling addict is aware he is addicted and wants to quit, rehab can be a solution, but if you want to slowly in his own way can also work, it's just that you should be more patient and should not easily give up.