Once again get a "real" electrician, if the GFI trips with nothing else connected and how do you know? I have a little device that plugs into an outlet and in this case you would have to bypass that gfi by installing a standard one, and reconnect all the disconnected ones. Now I can ID everything on that circuit by the tone with the other part of the tester. Saves disconnecting.
You might very well find what have disconnected are not, and those wires go to something else. Usually in those days 1999, only bathrooms, kitchens and outside outlets needed to be gfi protected.