Gyroscope is a function to adjust the viewpoint and aiming point in the game by utilizing the acceleration sensor of a mobile device.

When you choose "In a Scope Mode" in the setting, the gyroscope is activated only when you change the viewpoint to the scope attached to the gun. When you choose "Always" in the setting, the function is activated all the time.

But, when your mobile device has no gyroscope sensor, the function is not available. Please check with the manufacturer of the device for the availability of the gyroscope sensor.