In the recent update to Pokémon Go (Android version 0.42.2, iOS version 1.11.2), changes were made to the way training gyms worked. According to the patch notes:
Trainers can now bring six Pokémon to battle at friendly Gyms. The CP of the Pokémon you are battling may be temporarily adjusted lower for your training session.
How is this determined? I have attackers ranging in CP from the mid-1000s to low-2000s. Will the temporary adjustment only take into account my highest CP Pokémon overall, or will it adjust depending on what I have selected to take into battle with me?