General Feedback Theorem - Spice model and loop gain
It seems that in LTspice the .step param list command has changed.

So use the list parameter in increasing order: .step param z list -1 0 1

and change the sign in the expressions for Iz and Vz.

GFT.asc in the example folder of LTspice should work too.

And see the yahoo LTspice user forum.