Pokemon-go – How much prestige does a gym lose when battling it

A Pokémon will faint if they lose a battle while fighting at a gym, or if they get kicked out while defending at a gym.

If you get too many revives you can always dump them to make more room to hold other things.

The xp is 10% of the prestige that you get.

When training at a gym you will get 500 prestige per pokemon you defeat with equivalent CP value. ie. If you beat a 100cp pokemon with a 100cp pokemon, you will get 500 prestige.

You will get more than that if your Pokemon is lower than the one in the gym and less if it is higher.

How much less/more I am not exactly sure but as pointed out by Sobrique the formula may be P * D / A if the defending pokemon is of a higher CP. Where P is a multiplier of 500, D is the defender's CP and A is the attackers CP.

If the attacking Pokemon is higher then P is reduced to somewhere between 250 and 350.

The maximum you will get is 1000 prestige if your Pokemon is at least half the CP of the one in the gym. The minimum is 100 prestige which is when your Pokemon is double or more of the CP of the Gym's pokemon.

You cannot lose prestige by training at a friendly gym.