What Were the Trainer Classes of the Gym Leaders in Pokémon?

Everyone knows the Gym Leaders in Pokémon, and joining their ranks is one of greatest achievements in the world of battling! However, just as WE never start the games as a Champion, it stands to reason that each of these Gym Leaders started as another Trainer Class, perhaps one that we run into every day! And some of them may not be a simple as you'd expect...