Go to any forum on martial arts and you are bound to see someone asking the question, "What is the difference between kung fu and gung fu?" The answer is simple...nothing. This is like saying what is the difference between pop and soda? The difference is dialect and what part of the country your are from.
Kung fu comes from the Mandrin dialect; which is prominent in Northern China. Gung fu is Cantonese from Southern China. Both are translated to the same thing; "hard work" or "effort."