How to use gold finishing to reduce corrosion on a moisture sensor?
Many of the “hobbyist” soil probe circuit designs actually pass DC current through the probe, which inevitably results in electrolytic corrosion regardless of the metals used. If you only briefly power such a horrible simple circuit and with a low duty cycle you will get less corrosion per unit time. There also should be no flow to ground when off.

Gold will help with corrosion when not in use (you can bury a lump of gold for 100 years and it will look the same when dug up) but not with electrolytic corrosion, in fact it would just reverse the process by which thick layers of gold are sometimes applied (electroplating). The layer of gold on an ENIG PCB is less than a tenth of a micron in thickness typically so it doesn’t take much to strip it down to the nickel barrier.