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.