Pokemon gyms are team-based objectives where the winners will be rewarded with valuable coins. Gyms can hold up to 6 different kinds of Pokemon, but you can only place 1 Pokemon from yourself in that gym.
If the gym is your colour, you could add a Pokemon, start defending it, and make free coins. This can be done by pressing the + icon on the right bottom of the phone screen. If the gym is not your colour, you would have to start fighting to defeat the Pokemon inside the gym, before you can place a Pokemon inside that gym.
Sometimes, it’s impossible to see what kind of Pokemon is standing in the gym already. This is mostly due to an event being happening at the spot. Once this event is over, you could check again if there is still a spot, or if you would have to fight for it.
Remember that your Pokemon needs to be fed occasionally if you have food for him if his CP becomes low. Otherwise, it’s an easy walk in the park for other Pokemon Go trainers to reclaim their gym.
