Yesterday I gps-spoofed my char to a crowded place with a gym nearby.
The leader changed like every 3 minutes so I knew people are fighting there.
I discovered something that (hopefully) could be a bug:
If you just wait for 2 people to end their fight and the attacker wins, the Gym changes its color to white for a short time. That means that the leader is defeated and the attacker can place his own pokemon in the gym. BUT: most of them start to fill their pokemons HP first because it must have full life if you want to place it in. In this short time you can just place your own pokemon in the gym without fighting.
I just placed my CP 1200 Snorlax without fighting anyone