Here is a great video from the Youtube Channel Team Atlas Go. He provides a detailed overview of the current Gym Level Progression system and the requirements needed to reach higher Gym levels. Check out the video below and be sure to subscribe to his channel!Note: Unfortunately the Video has been taken down, but the synopsis can be read below:
Here is a breakdown of the video from beginning to end:
Level 1 Gyms start out with 0 XP. A Gym will be at Level 1 until it reaches 500 XP.
Deploying a Pokemon to defend a friendly Gym will increase that Gym's XP by 500, thus increasing that Gym from Level 1 to Level 2.
Level 2 Gyms require 500 - 999 XP and provide two total slots to for defending Pokemon.
Deploying a second Pokemon at a Gym will increase the Gym's XP to 1000 XP, thus increasing that Gym from Level 2 to Level 3.
Level 3 Gyms require 1,000 XP - 1,999 XP and provide three total slots for defending Pokemon. Adding a third Pokemon will increase that Gym's XP from 1,000 XP to 1,500 XP.
The remaining 500 XP needed to reach 2,000 XP and get to Level 4 requires players to train at that Gym. You must attack the defending Pokemon at that Gym with your own Pokemon (50 XP per battle).
Level 4 Gyms require 2,000 XP - 2,999 XP and provide four total slots for defending Pokemon. Gym level progression continues to work the same way as higher levels are reached.
Note: If you attack a Gym with two other friendly players at the game time (3 vs. 1) that one defending Pokemon will take damage from all three Pokemon. However, to make things fair the defending Pokemon will deal three times as much damage to all three attacking Pokemon.
When a Gym is defeated it will lose 250 XP. Once a Gym is defeated enough times and its XP reaches 0, only then will the ownership of that Gym be transferred to the winning team.
Gym Leader ownership can transfer when a Gym is defeated.
For example, if a Level 2 Gym is defeated and it goes down to Level 1, the defeated Pokemon from the second slot will be transferred back to its owner. The remaining Pokemon in the first defending slot will remain and the owner of that defending Pokemon will become the new Gym leader. If there are multiple Pokemon remaining at a Gym after being defeated and losing a Level, ownership will change to the Trainer with the most powerful remaining defending Pokemon.
Note: If your Pokemon is defeated you will not lose it. It will simply be transferred back to you.
Being that the current Gym system makes it much easier to take down a high level Gym than to build one up, choosing to deploy your best defending Pokemon is critical! Pokemon Go is still in beta testing, so the XP requirements may change. However, the current mechanics and overall system will likely remain in the official release.
I do not own this video. If you liked this video please be sure to check out Team Atlas Go on YouTube.