Saying gambling is addictive but not a medical disease begs for definitions of “addiction” and “disease.”
The essential element of addiction to gambling is that people become completely absorbed in the activity, which they then pursue in a compulsive manner, leading to extremely negative life outcomes. These individuals often describe a loss of control, such that, they claim they are incapable of avoiding gambling or of stopping after having started. All of these elements of addiction are behavioral, experiential, and phenomenological.
The disease model looks to an inescapable biological source for addictions, such that some neurochemical adaptation accounts for the observed compulsive behavior. In addition, a disease model would indicate, these neurochemical adjustments lead to measurable tolerance and withdrawal. Because the biological systems underlying the addiction are thought to be irreversible, the disease model includes the idea of progressive worsening of the addiction, with the requirement that some kind of treatment is necessary in order to cease addiction.

An Experiential Model
Obviously, an experiential model of addiction is much easier to observe. All it requires is that people sacrifice their lives to gambling and they assert or believe they cannot resist the urge to do so.
Evidence for this model is provided nightly at Gamblers Anonymous meetings, where compulsive gamblers sincerely attest how they have sacrificed everything to their addiction and have absolutely no control over their habit.
On the other hand, all of the elements of the medical (disease) model of addiction can be questioned, and in many cases have been explicitly disproved. Where addiction theorists and gambling researchers make their mistake, is that while finding compulsive gambling to fall short of attaining the status of medical disease, they discount its genuine addictiveness. In doing so, they often have assumed that alcohol and drug addiction fulfill criteria for an addictive disease that in the past, gambling has failed to meet.
Now clients coming into treatment fit the addictive disease concept of gambling behavior. Some people have extremely destructive experiences and develop chronic gambling habits and problems. The individual loses more than they intended, feels remorse, tries to recoup the losses by continuing to play, only to lose more  — good money follows bad. In direct parallel to the principal varieties of alcoholism, the person may fail to control any individual gambling experience in a gambling binge, or else may gamble steadily and heavily over a long period, forming a chronic dependence.
While the risk of gambling or prospect of winning can be exhilarating, the aftermath of losses, as well as being emotionally deflating, becomes increasingly problematic from a legal, career, and family standpoint. At the same time, future gambling relieves the anxiety, depression, boredom, and guilt that set in following gambling experiences and losses.
This addictive cycle is described repeatedly in the gambling literature. One critical element of the pathological gambling experience is money — the problem cycle begins with “negative feelings associated with gambling losses,” in addition to the “person’s positive experience of the gambling activity itself, shortage of money and the need to keep their gambling a secret.”
Addictive gambling resembles compulsive shopping, where people spend irrationally and accumulate debts and problems resulting from efforts to hide and/or cover up debts. The individual who is lost in this cycle is one who relies on magical solutions — as do drug and alcohol abusers to produce desired outcomes without following systematic or functional plans to achieve his or her goals.
Treatment and Gamblers Anonymous meetings are available if you or someone you know may have a problem.