Definitions for g-manˈdʒiˌmæn
Random House Webster's College Dictionary
an agent for the FBI.
Origin of G-man:
1920–25; prob. repr. Government man
G-man, FBI agent, government man(noun)
a special law-enforcement agent of the Federal Bureau of Investigation
A US government agent fighting organized crime in the interwar years.
A fictional character based on this definition, eg, Dick Tracy.
Origin: Probably abbreviated from government man