I'm still not sure I understand why the ground probe needs to be plugged into GFCI. It's the actual electrical equipment that needs to be plugged into GFCI, not the ground probe, right?Theatrus is right, but for more detailed info checkout post #1 here: https://www.reef2reef.com/threads/electrical-safety-gfci-afci-ground-probes.215701/
Here is the relevant part about why using a ground probe without a GFCI is dangerous: