So I defeated a Team Valor gym earlier today using my highest cp Pidgeot. It turned grey, so I went to claim it with that Pidgeot, except it wouldn't let me. What determines which pokemon you are allowed to place at a gym? Is it just those who have participated in a battle can't? Is there a time period for it? What about training at a friendly gym? Does this impact anything?
Your pokemon needs to be:
-Not actively in another gym
-Keep your best pokemon of each type with you, put the 2nd best ones inside gyms.
-Try to put pokemon that are not weak to the pokemon types in that area. For example, do not put water pokemon in gyms where there is a lot of electric pokemon spawns. Or you'll lose that gym immediately.