also worth noting is that the southern chinese dialect sounds more like "gung fu" while in the northern more staccato inflection sounds like "kung fu".
Kung Fu and Gong Fu mean the same thing. Gong Fu is Mandarin. Kung Fu is the English translation for Gong Fu, I guess, like Gongbao Chicken is translated as Kung pao Chicken.
Basically, any time you're trying to translate a language based on a completely different system into our alphabet you're gonna get every possible variety of spelling you can imagine. Add that to the confusion added by the different dialects used in china and you've got one heck of a mess.