G-men or G-man is gangster slang for FBI agents, meaning Government Man.
Gangster "Machine Gun" Kelly reputedly coined the nickname while being interrogated. The term's popularity increased in the 1930s after the release of James Cagney's 1935 film of the same name .
The G-Man is also referred to as the mysterious agent who observes Gordon Freeman in the Half-Life series of video games.
Source
