When we talk about "magnetic field" zone, we talk about
near field region of coil (or antenna), in fact
reactive part of near field, where primary changeable magnetic field (magnetic induction) act. Reactive part of near field is effective region of metal detectors, and, as we know, pretty limited in effective distance.
All other field (after transition zone) is
far field, where magnetic induction caused by changeable magnetic field, disappear. In far field region we can count only on resonance or on reflexion.
To get resonance with target at 30kHz (about GG working frequency) we need substantial part of Lambda (wavelength - which is here about 10km). At 30kHz we can get useful resonance (mean: re-radiated energy) with objects like underground cables, metallic pipes and alike. A hoard of coins, golden statue, etc. cannot resonate at such big wavelength.
So, using GG to detect something at say 5m, forget on magnetic field (magnetic induction) and forget on resonance. The only way is reflexive way (as per drawing above).
Without understanding how coil (which is sort of radiating antenna) works in near and far field region, proper use of devices like GG is impossible:
https://www.osha.gov/SLTC/radiofrequ...omagnetic.html