Gyms are an excellent source of EXP, especially in Fire Red Rocket Edition, where EXP and money are pretty scarce. That said, you can reach the credits screen without fighting a single Gym leader ...
As a child, becoming a Pokémon Gym Leader was high on my list of wishes ... While I intend to get this Pokemon leveled fully to be a Dusknoir/Goodra fusion, I’m not sure I’ll be excited ...