If you use copper ground rods use a brass fitting and copper wire to the ground terminal on the fencer. What they are worried about is the chemical reaction when galvanized and copper meet that over time builds resistance and reduces performance.

I have a Gallagher charger outside on a RR tie. I built a wooden box on the tie with an open bottom and sliding front panel. I ran the electric outlet up into it and the charger is in it. The material was some scrap 2x12 and a plywood front all caulked and coated with Thompsons waterseal.

You have to keep water away from the charger!   I would also suggest you consider putting a lightning choke and disconnect switch on the post. It is real inconvenient to have to pull a wet 120v plug to turn the thing off!

I have also gone back and forth about using a GFI outlet. I have had several times where the GFI has tripped by a storm and the charger is off. I no longer use a GFI on the charger.

Good luck.

Jim at Dawn