It's not necessarily a disease but more like a drug. And just like most other drug, you can get addicted to it. And that would mean you would be spending all your time and effort to gamble which is bad for you.
A disease on the other hand is more like a disorder or a malfunction of a human part.
Addiction is one of the diseases that must be treated, not only in gambling, all kinds of things that are addictive, such as addiction to drugs, sex etc. are diseases. So addiction is not good and you have to treat it and leave your habit that has become addicted so that you are free from the bond if it really makes you tormented.
Drug addict can be cured by taking medical treatment but for gambling addiction their is no medicine to cure it, it is only cured by the will of the gambler who got addicted or until he is totally not lost all his investment he wont realize what he has lost in his life.