As a korean I find this to be one of life's funnier things.
I only learned about this term some years ago and was incredibly amused.
Now, I might be wrong but my knowledge and a friend's knowledge and of my mother's knowledge of the word gook point to it meaning soup in korean. My mother was raised in korea until the age of 22 and is now 46. She still speaks korean better than english. I consider the translation to be very reliable.
Gook = Soup
Their are many better insults in the world.
What kind of soup?