Battle
From Pokemon Go Wiki
For this to happen, you need to go to a Gym. You can battle against friendly Pokemon for training or you can battle rival teams’ Pokemon to try to claim the Gym for your Team (Instinct, Mystic, Valor).
Defeating the rival teams’ Pokemon would decrease their prestige and Gym level. Once their prestige reaches zero, your team would now own the gym. Once you successfully claimed a gym, you can leave one of your own Pokemon to battle with other Pokemon and defend your team’s Gym.
