To activate Gym Mode, tap and hold the screen for 10 seconds.
This will hide the UI (menu and joystick) so it does not get in the way during Gym Battles.
Donors have an option to hide the blue gym mode button that appears at the bottom.
To enable this, go into PokeGo++ settings and activate the toggle switch.
If you wish to show the UI again, hold the screen for 10 seconds again and it will pop up again.