Both "govenor" and its adjective form, "gubernatorial", originally derive from the same Latin word "gubernare" (to govern) yet we use root "govern" in all contexts ("govern", "government", "governor", "governing", "governmental"...) except for "gubernatorial."
Why is "gubernatorial" the odd man out?
Also, is this just an American thing? In Australia, we also have governors but I've never heard the word used outside of the context of American politics.