The Collaborative International Dictionary of English v.0.48:

G-man \G-man\ n. [from Government man.]
   A special law-enforcement agent of the Federal Bureau of
   Investigation.

   Syn: FBI agent, FBI man, government man.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

G-man
    n 1: a special law-enforcement agent of the Federal Bureau of
         Investigation [syn: G-man, FBI agent, government man]