I am predicating my answer on a definition of “gambling disease” as meaning your referring to professional players that could not pass the 20 questions from the gamblers anonymous test or could be clinically diagnosed as compulsive gamblers. It has been almost 35 years since I played my firsthand of poker and had my first job in a casino, I base my answer on this experience.
My estimate of how common gambling disease is with PPP (professional poker players) is 99.5% give or take half a point.
This is not to say by any means becoming a PPP is a terrible thing for everyone, or even the majority of people that give it a shot. For a few it is the best thing that could ever happen to them. For most they go broke or they just decide it is not for them. Some exit the game via a 20th floor window or some other hideous manner. There are really as many stories as there are players. Some good, some bad, most way inside the extremes of the spectrum.
IMHO the one thing all PPP have is an addiction to the game of poker. They are compulsive about it. I do not believe you can stay competitive at the game at a worthwhile level unless you are compulsive and addicted to the game. Few successful PPP players I know would pass a GA test, or fare to well with a professional clinical evaluation of their gambling habits. It is not necessarily degenerate gambling, but it is the one common behavior that I have seen in every PPP player that I have come across in the past three decades or so. This compulsive behavior is not uncommon in most other professions. Indeed, most people whom are compulsive about their profession have the drive and focus to rise to the top and poker is no exception to this.
You need the compulsion to motivate you to get out of bed to play, to help you stay focused during long tournament sessions, to keep your game competitive and all those other things. For any PPP it is a fine line to stay in the zone over the years. Nobody is perfect about staying the things that make you excel over other players. Being addicted and compulsive with our game is like riding a bull, its easy to fall off. If we're really addicted, we really do not have a lot of control over it. Ironically most of us are not really addicted and compulsive enough to keep us competitive enough to make a living with poker. Unfortunately, some of us can’t handle the demon and become degenerate… And of course, some people are just to stupid to be PPP. At any rate I have never met a PPP that is not addicted to the game.