Background Gamma-hydroxybutyrate (GHB) use among gay and bisexual men (GBM) has increased in recent years. It is commonly cited as a sexual-enhancement drug. There is, however, little evidence for factors associated with GHB use or the consequences of its use among GBM. Aim: Factors associated with GHB use, its relationship to sexual risk behaviour, and the contexts, consequences, and motivations for its use were examined. Methods: The Following Lives Undergoing Change (Flux) Study is an online prospective observational study of Australian GBM. At baseline, a total of 3190 GBM provided details about their use of GHB. Data on frequency, methods, pleasures and consequences of their drug use, alongside key demographic variables were collected. Results: Mean age was 35.0 years. One in five men (19.5%) had a history of GHB use and 5.4% reported use within the past 6 months, with 2.7% having used it monthly or more frequently. Overdose had been experienced by 14.7%, this was more common among men who used GHB at least monthly. Being HIV-positive, having more gay friends, greater social engagement with gay men who use drugs, a greater number of sexual partners, group sex, and condomless anal intercourse with casual partners were independently associated with GHB use in the past 6 months. Greater social engagement with gay men who use drugs and group sex were independently associated with at least monthly use. More frequent GHB use was independently associated with experiencing overdose among GHB users. Conclusion: Most men used GHB infrequently and it was often used explicitly to enhance sexual experiences, often in the context of intensive sex partying. Men who used GHB frequently, were at greater risk of overdose and other negative health outcomes. GHB use should be considered alongside other drugs that have been implicated in sexual risk behaviour and HIV transmission. Harm-reduction interventions need to consider the particular impact of frequent GHB use.