Gambling disorder (GD) is a behavioural addiction that can lead to negative outcomes. Factors such as the type of gambling and a person’s sex might influence the duration of GD. This study defined duration as the time between the onset of GD and beginning treatment. It examined how duration of GD is related to treatment outcomes (i.e., relapse and dropout). Participants were 1,699 patients who met the DSM-5 criteria for GD and sought outpatient cognitive-behavioural therapy in Spain. Participants who engaged in strategic gambling were more likely to have a poor treatment outcome in the first five years since GD onset than those who engaged in non-strategic or mixed forms of gambling. Women were also at higher risk of poor treatment outcomes than men in the first years after GD onset.