A sketch of what you are trying to accomplish may make this easier to understand.
To be protected and have a GFCI work, if you plan to use the 'Load' leads (i.e., feed anything else from that GFCI receptacle), the neutral for the load side can ONLY be connected to the items fed by the hot from the GFCI load side, and not to any other neutrals from any other circuit.
A GFCI works by comparing the current between the load hot and neutral. Any current flowing from another circuit will unbalance that, and cause it to trip.