At the service panel, the ground and neutral are bonded together if there is a separate ground bus, and grounds and neutrals are connected to the same bus if there is only a neutral bus. GFCI breakers compare current into the hot and out of the neutral. Any difference is assumed to be going through an unintentional ground (like you, if you are about to get electrocuted). Most likely you have a bad section of wire that is leaking current to ground.