No others loads on the grounded conductor(neutral) that is on the load side of the GFCI breaker. The GFCI breaker is measuring the current on the load side ungrounded(hot) and comparing it to the current on the grounded conductor and the two currents must be equal.
Greek If you need to use a multiwire circuit and have GFCI protection it would require that the GFCI's would have to be placed after the multiwire circuit split into the two circuits which would probably mean using a GFCI receptacle. They do make a NM with 4 conductor and ground but it's not cheep. and the other way is to use a two-pole GFCI breaker but that cost more too.
But to add to what stamcon has said if the current is higher on the ungrounded conductor (hot) than is on the grounded conductor (neutral) it will cause the GFCI to trip. and with a multiwire circuit, The neutral will have the differential current on it meaning the neutral will have less current than the hot's so the GFCI will sense this as a ground fault and open the circuit.
Also this apply's to AFCI breakers also.