Sociodemographic background factors, gambling game type, depressive symptoms and comorbid substance use have been associated with gambling disorder, but the associations of these factors with treatment outcomes remains unclear. This study examined background factors in participants of a clinical trial, identifying factors which were associated with treatment outcomes such as gambling urge and quality of life. This was a secondary analysis of a single-center 12-week randomised placebo-controlled clinical trial investigating the effects of as-needed intranasal naloxone combined with psychosocial support in the treatment of gambling disorder. Independent variables were analyzed with linear mixed models, with analyses both including and excluding treatment allocation. Sociodemographic factors or game types were not associated with treatment outcomes. Adherence to trial medication and high self-perceived readiness to change gambling behaviour were associated with greater reduction in gambling urge and severity, and greater increase in gambling self-efficacy and quality of life. Higher scores for depressive symptoms were associated with more severe gambling and a slightly greater decrease in gambling urge, as well as a greater increase in quality of life. When included as a nuisance factor treatment allocation abolished most statistically significant results. Participants with high medication adherence had improved treatment outcomes compared to participants with low medication adherence, possibly representing greater motivation and commitment to treatment. More severe depressive symptoms were associated with a greater reduction in gambling urge but not gambling severity.