Internet shorthand for Matthew Gray Gubler of Criminal Minds fame, often used on livejournal.
Occasionally written as em-gee-gee if the writer is feeling like a giggly fourteen year old girl.
In other news, MGG is a total GQMF.
Mammary gland Gyration... the act of tittyfucking
MGG is the same as tittyfucking, just a better way of saying it