Gamma butyrolactone (GBL) is a chemical. People use it as medicine. Be careful not to confuse GBL with gamma-hydroxybutyrate (GHB) or butanediol.
Despite being illegal and carrying serious safety concerns, people use GBL as a recreational drug or for purposes such as sleep, sexual performance, and many others, but there is no scientific evidence to support these uses. Using GBL can be unsafe and cause severe side effects including addiction, withdrawal, coma, and death.