The GP3500 is still highly regarded down here in Australia. The last of the true analogue PI gold detectors. They are maybe a bit more prone to EMI than the newer models so the mono coils can be a bit noisy in urban areas.
Tune it in regularly (noise cancel) and maybe hook up a DD coil if EMI is a problem. Lovely audio and packs a punch for sure.
I'm sure it will serve you well.
HH,
Tony