Gambling disorder (GD) is a highly heterogeneous condition with high rates of chronicity, relapses and treatment dropout. The aim of this study was to longitudinally explore the associations between trait impulsivity, impulsivity-compulsivity related cognitive domains, and treatment outcome in an outpatient sample of adult patients with GD.144 adult male participants diagnosed with GD undergoing cognitive-behavioural treatment (CBT) at a specialized outpatient service completed a series of neuropsychological tests to assess executive functioning (including cognitive flexibility, inhibition control and decision making) and psychometric questionnaires.Trait impulsivity predicted low compliance [UPPS-P negative urgency (B = 0.113; p = 0.019)] and relapse [UPPS-P negative urgency (B = 0.140; p = 0.015)] at 5 weeks of treatment and dropout at the end of treatment [(UPPS-P sensation seeking B = 0.056; p = 0.045)]. Cognitive flexibility performance predicted: dropout rates at the end of treatment [WCST perseverative errors (B = 0.043; p = 0.042)]; dropout [WCST categories completed (B = −1.827; p = 0.020)] and low compliance or relapses at follow-up [WCST perseverative errors (B = 0.128; p = 0.020)]; and time to first relapse [WCST failure to maintain set (B = −0.374; p = 0.048)] and time to dropout [WCST perseverative errors (B = 0.0198; p = 0.019)].Our findings indicate impulsivity-compulsivity levels may influence response to GD treatment (i.e.: low compliance and dropout or relapse rates) thus representing a potential target for improving treatment outcomes.