After a lot of work, I finally got an acceptable continuous GCM power-gate test circuit to work.

The primary side is largely unchanged from the old relay test system. From the AC inputs a pair of rectifier diodes turn the AC into pulsed DC. A large flame-proof resistor reduces the current down to ~1.5mA or so, which is sent through the primary side of an optoisolator. A smoothing cap is across the opto as well.

On the secondary side, there's a cap that is charged through a charging resistor. The secondary of the opto serves to discharge that cap. The result of that goes into a schmidt-trigger buffer to give the circuit some hysteresis. The output of the buffer is fed into the gate of a P-MOSFET that sends power to the logic board, and is also used to light a FAULT LED when the gate is held high.

The result is that the GCM circuit will inhibit power to the logic board when the ground is not in an acceptable range. Initial tests put the high end of the acceptable ground impedance at around 13 kΩ or so, which is inside of UL's desired range of 100Ω/V.