Can you kick a Pokémon from a rivaling Gym by lowering the Gym level?

Yes, if you defeat a Pokemon multiple times and the Gyms level goes down, the weaker ones get kicked out.

There are prestige levels which gives the gym it's level, and you can have 1 slot per level. Also adding a pokemon to a gym gives +2000 prestige.

• 2000
• 4000
• 8000
• 12,000
• 16,000
• 20,000
• 30,000
• 40,000
• 50,000

if you go pass a level, the number of slots will change and kick the weakest pokemon if needed.

Exemple a Gym at 8200 prestige with 3 pokemon, looses 2500 and goes down to 5700. It is no longer lvl3 but lvl2, and the weakest pokemon (first on in order of CP) is removed.