This is a department exception to the requirements in 2011 and 2017 NEC 547.5 (G): Exception: Ground-fault circuit-interrupter protection is not required for a single receptacle providing power for an electric fence controller used for livestock containment where an accessible ground-fault circuit-interrupter protected receptacle is located within 900 mm (3 ft) of the non-GFCI protected receptacle.
Wis. Admin. Code Department of Safety and Professional Services SPS 316.547