Verify that the gfci breaker matches the circuit wiring. This 20 amp 120 volt breaker is a form of gfci that can be installed at the circuit source. Receptacle Schematic Wiring Diagram 2 Wiring Diagrams
A gfci outlet is different from conventional outlets. So gfci designed as checking the difference between the current leaving and returning through current transformer of the gfci to protect device exceeds 5ma. Circuit Breakers Abbindustrial