I'm from the North, and sometimes it would be referred to a 'Gee', the 'G' as in GO!
Probably more dialectic than anything else, really.
However me oul' boy used to ask me if I had a 'jig' tonight (pronounced as spelt). I thought he was just trying to be 'hip' to the lingo.
Should have asked him really.
'Fanny' is indeed the 'Gee'.
Maybe it's called 'Gee' because that may well be what one would say upon viewing it!
B.