To gain the rank of Gym leader for starters you have to have a specific type of pokemon for example, Rock Gym you will need 6 Rock Pokemon. That is not all, depending on what gym you want to be the leader of also depends on the level of your pokemon.
Rock Gym : levels 20+
Water Gym : levels 30+
Electric Gym : levels 35+
Grass Gym : levels 40+
Poison Gym : levels 45+
Psychic Gym : levels 50+
Fire Gym : levels 60+
Earth Gym : levels 70+
It may sound easy but your mistaken, training your pokemon will take time and effort not just sit there banging your keyboard and clicking random stuff.
