The alpha setting for the edge has no effect in the gnuplot terminal.  For example:

graphics_toolkit ("gnuplot")
figure
demo ("patch", 4);
pfid = get(gca, 'Children');
set(pfid, 'facealpha', 0.5);
set(pfid, 'edgealpha', 0.2);

See if that can be fixed so that the edge has alpha scaling, and if not try for something that turns off the edge with

set(pfid, 'edgealpha', 0);

because it would be too difficult to make the border appear nonexistent when attempt to match alpha blending.  One could set the color of the edge the same as the face, but can't match alpha.  Removing the edge seems a mandatory feature.