Gym Defender Pokemon will be impacted by the new motivation system.
Players will be able to spin the Photo Disc at all Gyms to collect items, just like a PokeStop. This update will add additional occupational slots to existing gyms so that each gym supports six slots in total.
As you progress through gyms across the world, you'll also be able to start collecting gym badges. The level of your gym determines the bonuses you receive while a part of it, so best to level it up as best you can.
"Raid Battles are timed in-game events that will temporarily take over a Gym, providing Trainers from all teams the chance to visit the Gym and group up together to challenge the Raid Boss and potentially earn special new items along with an opportunity to catch these Pokemon", the post continues.
Full details can be located at the link above, but this is definitely going to change the way the game is played and how people will react as a team to accomplish goals. Much like other Pokestops, gyms will attract wild Pokemon, but occasionally a raid boss Pokemon will spawn at a gym and supplant all defending Pokemon. You can get one free Raid Pass per day by visiting a Gym, but you can only hold one at a time. Unlike the previous system, all Pokemon must be unique. Pokemon will automatically lose motivation over time as well as when they are defeated in battle. As a Pokemon loses motivation, its CP will temporarily decrease, making it easier for opposing teams to defeat.
As well as these in-game events, you'll also be able to create Private Raids, so that you can run these battles with your friends. Item acquisition will be possible by spinning the Photo Disc and accessing Gym-based features and content won't be dependent on Prestige and training. However, a Pokemon's motivation can be increased by feeding it berries, which may mean that we need to revisit gyms in person to feed our Pokemon.
There will also be 6 permanent slots for gyms (although it's not clear if this changes as the Gym levels up), and rather than battling the controlling Pokemon in order of CP, you'll now battle them in the order that they joined the gym.