Recent technological developments have brought about notable changes in the way people gamble. The widespread use of mobile Internet devices and gambling websites has led to a significant leap in the number of people who recreationally gamble. However, for some, gambling can turn into a psychiatric disorder resembling substance addiction. At present, there is a shortage of studies examining differences between adults with gambling disorder (GD) who exclusively make sports bets online, GD patients that are non-sports Internet gamblers, and offline gamblers. Therefore, this study was undertaken to determine the differences between these three groups, considering sociodemographic, personality, and clinical characteristics. The sample consisted of 2,743 treatment-seeking male patients from the Pathological Gambling Unit at a university hospital. All patients met DSM-5 criteria for GD. We found that gamblers who exclusively engaged in non-sports Internet gambling activities were younger than offline gamblers and online sports gamblers. Non-sports Internet gamblers were also more likely to have greater levels of debt compared with offline gamblers. In terms of personality characteristics, our sample displayed low levels of self-directedness and cooperativeness and high levels of novelty seeking. In addition, online sports gamblers obtained higher scores in persistence than non-sports Internet gamblers and offline gamblers. Although differences if terms of gambling severity were not identified between groups, GD patients who exclusively bet online appear to possess distinct personality characteristics and higher debt levels compared with offline gamblers.