To become a Gym Leader, you must first obtain 20 KO's and 60 Trainer Points, and have a themed squad. Whereas previously you were required to own at least 6 of one type of Pokémon, this is no longer necessary, and will lead to a broader scope of Gyms. Although most themed squads will contain at least 6 Pokémon of one type anyway (Knobert's Gym Squads for example), this rule does not change much. The best example of a themed squad is pokemongerGS' "Team Eevolution", a squad consisting of Eevee and all its evolutions. Being a Gym Leader entitles you to a special benefit, as outlined further below.

There may be 17 type gyms (one for each Pokémon type) plus any number of theme gyms.

Gym Leader Rules

Once a trainer satisfies the requirements to become a Gym Leader, they have the option to choose which Type or Theme Gym they want to run.

There can be at most 2 Gym Trainers at any Gym. These trainers will be trainers that wanted to become Gym Leader but didn't earn the requirements fast enough. They will be accepted to the gym at the Gym Leader's discretion.

Gym Leaders and Trainers will have 3 more slots added to their current maximum size. Gym Trainers, however, will not have the 1 less badge or 1/5 off the points requirement to advance to the next Trainer Level.

The Gym Leaders will be responsible for describing the battle rules of their match AND their Gym Trainer assistant matches.

A trainer can only be on one Gym Leader's queue at any one time.

Scenario

A new Gym Leader will have 3 mandatory battles, regardless of how long it takes.

Upon completion of the 3 battles:

If the GL won at least 2 out of 3: He/She can stay on as GL for another 3 matches.

If the GL doesn't satisfy this, then they may be removed from their position at the discretion of the League Officials. If the Gym Leader loses their position, they may be replaced by a Gym Trainer or the Gym may become empty and available for another trainer to take.

Also as in the case of League Champions losing their title, if you lose your Gym Title, you will still hold benefits because of the possibility of level drop and/or Pokémon "de-evolution".

Gyms & Gym Badges

To gain a Gym Badge, you must challenge and defeat the leader of a gym (obviously). You may be required to defeat, or at least face, one or more Gym Trainers (within reasonable limits) before you are allowed to challenge the Gym Leader however.

Each Gym has a fixed arena created by the Gym Leader. These arenas may have special rules for Wild Pokémon, and special events that may be activated accidentally or on purpose by one of the trainers.

All Gym Leaders are automatically assumed to have their own badge, and so automatically gain the benefits associated with it.

The League Champion

After the conclusion of each Tournament held in the league, there may be a League Championship match.

The League Champion position can only be obtained in a special League Champion match as decided and held by the League Officials. Being the League Champion entitles you to a special benefit.