https://www.polygon.com/2018/10/11/17959930/pokemon-go-blissey-gym-stats

Competitive Pokémon Go players are anticipating a major, upcoming upheaval in the battle scene, thanks to Niantic’s announcement that it intends to rebalance the game. The developer has already hinted at what kinds of changes it plans to make — and it sounds like they’re aimed at kicking the best gym defenders out of their top spots.

“Changes to Pokémon effectiveness in battle” that Niantic is working on include adjustments to HP, defense and stamina points. Those last two are the most crucial for players who are used to maintaining control over gyms indefinitely, thanks to the Pokémon they’ve left to defend it.