When I was younger, I frequently wondered why we say gubernatorial instead of governatorial. The answer lies in the Latin term gubernator, from whence both words come. This originally meant "commander of a ship" but got figuratively extended to other types of commanders, such as those of provinces. Later, that evolved into Old French gouvreneur (the b to v switch happening because the bilabial plosive got confused with a bilabial fricative, which then got confused with a labiodental fricative - this is called betacism), which became English governor. Gubernatorial, meanwhile, was borrowed straight from the Latin word, and that's why they're spelled differently. Gubernator traces to the verb gubernare, which meant "to steer a vessel", which is from Ancient Greek kybernan, which has an unknown origin.
Adam Aleksic, a rising sophomore studying linguistics and government at Harvard University, has been described as the internet's sixth most famous etymologist. He also has disturbing interests in politics, vexillology, geography, board games, conlanging, and law.