1) An expression of excitement: it is appropriate to say it when a fitty walks past you, especially in the presence of friends, to ensure that she does not understand what you are saying.
2) To encourage your friend to talk to a girl: sometimes a girl will look at your mate but, instead, they're too nervous to make conversation; by shouting 'GITM' at your mate -- one letter at a time -- it will inject them with a sudden rush of confidence
3) More loosely: for when anything good happens!!!
2) Mannn that girl is well into you, can't you see her looking at you?...GITM!
3) That was a banging goal...GITM!; GITM!...I've no work today because it's the weekend; GITM!...I just won the lottery!