Each pokemon gym operates with both similarities to all the others, and quite a few differences as well. Each gym specializes in a single pokemon type. Each gym has 1 leader and 2-6 gym trainers.
Each gym contains 2-6 Gym Trainers that must be all defeated before the gym leader can be challenged. gym trainers have a varying range in their skill levels.
Gym Leaders operate differently. Each Gym leader has a fully realized team of fully evolved and trained pokemon. But they do not use them all the time. they will use the unevolved forms of their strongest team as replacements when facing trainers with few or no gym badges. So for example if a gym leader had a charizard in his strongest team, then he might use a charmander when he is facing a new trainer. They will adjust the strength of their pokemon to provide a good challenge to any would-be challengers.