What do you think are the requirements to be a Gym Leader?
Well, it has never really been explained how you do become a gym leader, neither in the games or whatever else. There has been appearances of several smaller, unofficial gyms throughout the anime, so with that logic, there aren't actually any requirements to become a gym leader, as you can just start a gym yourself xD However not official :p
In all seriousness, this is a topic where you can perspectivize a bit with high-ranked jobs in real life. There are several ways to get to those. One is the most obvious way, where you work hard, put a lot of effort into your work, etc, and the same can be applied for the gym leaders. If they train hard, make themselves noticed in the world, and have decent Pokémon, they'll be able to work themselves towards gym leader rank. An example of this could be Misty. She's very young in the games, though appears to have worked hard to become a gym leader.
Another way is through connections. It's not all that often, but you do hear cases of people getting good jobs with absolutely zero amount of effort put into gaining it, only because they have a family member or a good friend up in the high. The same applies to gym leaders of course. Tate and Liza are both very strong, but I don't think that they could've become gym leaders without some help from a relative. I mean, compared to Misty, they are several years younger, though not a clear example. Janine also made gym leader through her father when he became E4, though I do think she'd have made it anyways.
Though yeah, even though some of the earlier gym leaders don't appear to have that strong Pokémon, they probably have stronger ones stashed away somewhere. In theory though, all gym leaders could be about equally strong, where levels are only put in to enchance the gameplay, though that is true in both cases, so that's not really anything to go on.