I have met many users who have a level in Pokémon GO higher than mine (it is what has to work), and who have many more Pokémon. But even so, almost none of them have focused on gyms. They leave it in the background, as if it were something irrelevant. Actually, it is one of the keys of the game, and it is the way to get free Pokécoins, without having to pay. And not only Pokécoins, but also other rewards.
Defend the gym
Basically each gym is dominated by a different faction or team. You are part of a specific team if you have already reached level 5. This gym has a specific level, and some prestige points that depend on the Pokémon that are defending the gym there. You can leave your Pokémon in the gym as one of those who are part of the team that defends it, as long as that gym is dominated by your faction. What do you get when you leave your Pokémon there?
For every day that your Pokémon is in the gym, you can claim a daily reward, a defense bonus, which you will find in the upper right corner of the store. For example, you can receive Pokécoins, for which you will not have to pay anything, of course. That’s the only way, right now, to get Pokécoins without having to pay. So, if you have a high-level Pokémon, instead of having it on your Pokémon list, go to a gym of your faction that seems resistant, and add your Pokémon there to defend it, for every day it is there you can request the reward daily.