If the Gym is white, it means it is unclaimed. Go and get it! You can then choose which Pokemon you’d like to leave to guard it. You’ll want something strong, as other trainers will challenge it. If a trainer from a rival faction defeats it, they will be able to claim the Gym for themselves. Be wary that if you want to continue battling other Gyms, it’s a good idea not to leave your only strong Pokemon in a Gym.
Originally Posted by: gamespot.com