Can I level up my Pokémon while they are at gyms?
I just dropped my Vaporeon off at the gym, but to my surprise, he can still be viewed in my Pokémon list! The buttons are there to level him up, but I'm not gonna be able to get any Eevee candy for a few hours to try. Can I level him up while he's in the gym?
You can not level up or evolve Pokémon while they are at a gym until they have been defeated and returned back to you, fainted with 0 HP.