See DSM. Essentially, when instead of the pleasure it becomes problematic. The person is preoccupied with the habit; spends more and more time gambling; when is not gambling, is irritable, unhappy; using of gambling instead of other responsibilities; the gambler is chasing his losses; there is no control over the habit; the consequences – legal and financial from gambling, etc. I treat is with "extinction.
Troubles. Gambling addicts are unable to control their gambling in spite of negative consequences in their work and family lives. Mutual support helps: gamblers anonymous for the gambler, gam-anon for their families and friends.