I'm sure you'll get a wealth of replies and to your post
from folks who are much more knowledgeable than me. One thing that I
do know is that gold filled wire generally should not be soldered. It
is meant for wire wrapping, which normally means no soldering or
gluing is involved. The reason solder is normally not done on gold
filled wire is the way gold filled wire is made. The gold layer is
pressed or rolled over the core. When heat is applied, the gold melts
off at a different rate than the core which basically renders a
pretty much useless mess.
Having said that, I've seen items that have been soldered and are
labeled gold fill. Toggles come to mind. Confusing at best.