It really depends on how gambling addiction take effects. If the person or people are having bad symptoms like, they start betting their personal properties, or simply destroying their lives, then you can say that gambling addiction is a disease to that person. But if the person's mind is still stable, then that means that gambling addiction to them are not a disease.
Yes, that's true too, because gambling addiction will also make a person feel depressed because they receive a lot of losses. That way there is also the possibility that they can do negative activities such as stealing etc. So as long as their souls can still be well controlled over addictions in gambling, then they are still in good sanity and not a disease.