No doubt in your saying that no one can quit gambling instantly for good. He cant quit it even if he wants to quit thats why call it as an addiction.
With strong willpower and determination to redeem one's own self an addicted gambler can even quit gambling in a few days. Of course the iron will has to be there and the hatred towards gambling needs to be present.
Its like the same that someone inhales oxygen while he wants to quit inhaling Oxygen because if he will do this, he would be soon dead.
Oxygen is essential for life. On the other hand gambling is an addiction or in other words oxygen is a necessity while gambling is an indulgence. Hence this is a poor comparison.
Gambling has a proper method with which it can be easily cured.
Gambling is not a disease. It is an addiction. Calling it a disease is an oversimplification of facts.
I agree with you in this regard that by having a strong will power the person can do anything but in case of gambling, I think it will take more time than usual for the person who is addicted to it to quit gambling. This is because the person becomes much greedy in that duration when he didnt thought to quit it and overcoming that greed is much difficult.