First, the gyms are sometimes glitchy when youre using data. I have network errors during battles and in between Pokémon. Other than this issue, the app isnt terrible. Just a headache at times.
Second, it would be nice to have a push notification reminder telling you when to collect your coins and stardust from PokéGyms. Also, a push notification alerting you that a Pokémon has been knocked out of a gym. Additionally, with more Pokémon in gyms, sometimes its difficult to remember where your Pokémon are. Maybe a feature telling trainers what gym their Pokémon are in would be helpful.
Totoxert about Pokémon GO, v1.31.0