The 'Gym Fight' contextual panel

This menu only has one panel:

This panel will only be shown if the player is currently in a town location that has at least one gym unlocked.
In case of multiple available gyms (the league for example), the drop-down list can be used to choose the contestant to fight.

Turning on the 'Auto Fight' feature will automatically enter the gym.
If you want an auto click-attack feature, use the one from the Automation panel.