Pokemon Brilliant Diamond/Shining Pearl: Guide for Defeating the Gym Leaders & Elite Four

I recently got Pokemon Brilliant Diamond, so I thought I would do a little guide on the gyms that you have to go through during the course of the game. There is no difference between the games and the gym leaders. They are similar to the classic Diamond and Pearl games, so you go through the gyms to face the trainers before reaching the leader. There is no challenge or mission, or special feature of the gym like there was in Pokemon Sword/Shield.