What does the "G" mean in "G-Man"?

The story behind the answer

Government is correct because “G-Man” is short for “Government man,” a nickname for U.S. federal agents, especially FBI officers. The term rose to prominence in the 1930s during the war on gangsters, when the public and the press used it to distinguish federal agents from local police.

A famous (though possibly apocryphal) boost came in 1933 when gangster “Machine Gun” Kelly allegedly shouted, “Don’t shoot, G-men!” during his arrest, cementing the term in popular culture. Think “G = Government” to remember it: a G-Man works for the federal government, not city or state authorities.