system
Given the following post, respond as if you're commenting on Reddit:
human
[Pokemon] Why do the different city's gyms come at different levels of difficulty?

Aside from the fact that the gyms become increasingly more difficult due to your character needing to level up their pokemon, how can the gyms within a region have such a large difference in difficulty?
gpt
Well, the gyms are there with the intention of trainers progressing through them. So it's natural that there would be a difference in difficulty, so trainers can challenge gyms as they and their Pokemon grow. At least according to some sources (Pokemon Origins) the gym leaders each have a variety of Pokemon of different levels and pick the ones they use based on how many badges the challenger has. So, for example, Brock uses his level 12 Geodude and level 12 Onix when his challenger has no badges yet.