What is the maximum gym badge tier?

Solution 1:

The following amount of Badge-EXP (BXP) is needed to level up your Gym badge:

Gym Badge Tier // BXP needed // BXP total

• Basic // 1 BXP
• Bronze // 500 BXP // 500 BXP
• Silver // 3500 BXP // 4000 BXP
• Gold // 26,000 BXP // 30,000 BXP

Received BXP rewards for interacting with the gym:

• Adding a Pokémon to the Gym (+ 100 BXP)
• Holding a Pokémon at the Gym (+1 BXP per minute)
• Defeating a defending Pokémon (CP of Pokémon defeated divided by 10)
• Winning a Raid battle at the Gym (+ 1000 BXP)
• Feeding a Berry to a Pokémon (+ 10 BXP & 20 Stardust)

Source: Pokemon Go source code

item_templates {
  template_id: "GYM_BADGE_SETTINGS"
  gym_badge_settings {
      target: 500
      target: 4000
      target: 30000
      battle_winning_score_per_defender_cp: 0.01
      gym_defending_score_per_minute: 1
      berry_feeding_score: 10
      pokemon_deploy_score: 100
      raid_battle_winning_score: 1000
      lose_all_battles_score: 5
  }
}