In a Private Servers the server owner can allow users to customize their gym. Customizations cost Money or Gems. P Readlayers can customize their own server, including building their own stuff in the server. Private Servers may also feature Arenas, as well as Gyms. Building can be done in the actual server. Private servers can be found in the server list UI in the game.
User customization
Users can customize private servers for usage.
Improved server limit
You can increase the server size by buying a developer product that unlocks larger servers