INTRODUCTION: Gambling disorder (GD) is a prevalent psychiatric condition whose severity is typically defined by the number of DSM-5 criteria met out of a maximum of nine. The relationships between the levels of gambling severity, thus defined, and other measures of psychopathology and everyday functioning are clinically important. METHODS: Baseline data were collected in patients with GD, conducted from 2001 to 2016. Participants completed clinical interviews and questionnaires. The impact of disease severity (mild, moderate, and severe) on clinical measures was characterized using analysis of variance models. RESULTS: The sample included 574 adults with GD, of whom 73 (12.7%) had mild, 184 (32.1%) moderate, and 317 (55.2%) severe GD. The moderate and severe cases, compared to mild severity group tended be older, had later age of onset, lost more money to gambling in the preceding year, had worse quality of life, had higher state anxiety and depressive scores, consumed more nicotine via smoking per day, and had lower venturesomeness scores. The moderate and severe groups did not differ significantly on these measures, however. The Yale-Brown Obsessive Compulsive Scale modified for Pathological Gambling (PG-YBOCS) discriminated significantly between all three groups. CONCLUSIONS: Several measures of psychopathology and functional impact of gambling symptoms appear similar between moderate and severe GD cases, while mild cases are clearly differentiated from moderate and severe cases. Thus, the current working definition of GD symptom severity boundaries has important limitations in terms of potential clinical utility.