<b><i>Aims:</i></b> The aim of this study was to evaluate posttreatment changes of individuals with a diagnosis of gambling disorder (GD) treated with group cognitive behavioral therapy (CBT), to assess the potential moderator effect of sex on CBT outcome, and to explore the best predictors of posttreatment changes, relapse, and dropout rates. <b><i>Methods:</i></b> A cohort design was applied with a prospective follow-up. The sample comprised 440 patients and the CBT intervention consisted of 16 weekly outpatient group sessions and a 3-month follow-up period. <b><i>Results:</i></b> Patients showed significant improvements in both the level of psychopathology and the severity of the gambling behavior. High self-transcendence and the involvement of the spouse or partner in the therapy predicted a higher risk of relapse. Younger age and low education predicted a higher risk of dropout. <b><i>Conclusion:</i></b> Many patients with GD can be treated with strategies to improve self-control and emotional regulation, but other techniques should be incorporated to address the individual characteristics of each patient. This is particularly important in group therapy, in which the same treatment is applied to several patients simultaneously. The involvement of a family member needs to be carefully considered since it may have a negative effect on the response to treatment if not adequately managed.