Guest Steve, the (sometimes derogatory) name "Gog", applied by a South Waleian to a North Waleian, derives from the welsh word, "Gogledd", meaning "north".

If I might be politically a little uncorrect here, in the last century a typical piece of mutual insult might go:

"Gwell bantu na hwntwr" (Better to be from Africa than South Wales) "Gwell wog na gog" (Better to be from the Indian sub-continent than North Wales)

This was quoted in "This sweet and bitter Earth", a novel by Alexander Cordell.