Pokemon-go – First challenger loses but team win means elimination

There are no fights before player level five. You will also not receive Potions and Revive items from Pokéstops before this level, as you have no use for them without access to fights.

Evolving can be helpful, although Combat Power (CP) is generally a much stronger indicator of success in a Gym battle. Additionally, you will want to build your team to combat the enemies' strengths and weaknesses. This follows the typical format from earlier Pokémon games (e.g. Fire is super effective against Grass, Electric is super effective against Flying, Water is super effective against Fire, etc.). This web page should be a useful resource in helping you build a strong team to counter your opponents' types.

The gym system is a little complicated, but in essence, by fighting an enemy gym you reduce that gym's prestige (until eventually your team takes it over). Once your team takes over a gym, you can increase its prestige by continuing to battle there. If there is an empty slot in the gym, you should be able to station a Pokémon there, which will help defend it from enemy teams. Once every 21 hours you can claim a reward for holding a gym by going into the shop menu and tapping the shield icon in the top right. The reward for each Pokémon that is still stationed in a gym is 500 Pokédust and 10 Pokécoins.

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.