Meaning God Fucking Dammit. Usually used in this form when in a public place, or somewhere where it is inappropriate to use extreme vulgar language i.e. Church, School, DayCare.
Example 1: I got an F on my test!? G Effing D!
Example 2: The Bible says i can't have sex until marriage!? G Effing D! (Gets struck by lightning).
Example 3: You kicked my kid in the head!? G Effing D lady!