My guess is No it dosn't need to be Manually Balanced,
because your asking.
I would Think you would see a Ground Balance Knob if it does.
On Manuals.
Contact Garret by EMail or Phone & request a Manual.
just Explain You are unable to Download one.
OK , I'm Correct :
From the Manual :
As the Detector Starts the Battery condition is checked audibly
and displayed on the LCD Display.
ALSO, During the Detectors Start-Up the detector adjusts automatically for the illimination of ground Minerals.
and the audio threshold is set to a point that is just silent.
the last used pattern of discrimination is re-established and
displayed on the lcd face.