OK, I've encountered an oddly wired outlet in my kitchen and am trying to replace it with a GFCI. I unwired the old outlet, taking note of where things were attached, just in case. I wired the GFCI acoording to instructions and then checked the wiring with a Sperry GFI checker. All seemed OK. Then I plugged the garbage disposal back in and it turned on (the switch was off.. ). Turning the switch off tripped the gfci.

Figuring I did something wrong, I retested and now find that I have an open neutral though none of the connections are loose - all have been rechecked. I did a bit of reading about open neutrals, especially since I only had three wires (plus a ground) to attach to the GFCI. This is where it gets weird and I need some advice.

--The hot side is a black Line and the *white* Load that has been marked with a black stripe with a pen. ( I tested and reversed this to neutral and the Sperry told me I had the wires reversed so I put them back). Also wire that should be the black Load has been ganged/attached to the Black Line.
-- On the Neutral side, I have one white Line, attached directly across from the black Line. I have no White Load, presumably because the other white wire has been made hot/black line and its corresponding black has been ganged to the Black Line, leaving me no other neutral option, I think.

I'm willing to call a professional if need be, but would appreciate knowing how to deal with this for future problems I may discover in my new (to me) home.

Thanks,
Robin

Sounds as if there is some issue with the third wire. Need to fiqure out what it is for. Any hot load wire needs a neutral, and this is not going to work as the gfi is seeing an imbalance due to something missing.

Need to test for 120 volt feed, and trace that third wire before we can determine what the problem is.

If you are unable to do the testing and tracing, may be best to call in a pro.