You were asking about varnishes for gum prints.
If you are looking for glossy blacks in future prints, why not use a stronger
black, say indian ink, with Gloy as the medium, and then you would have no need
for a varnish.
This method, but without the Gloy, worked astonishingly well for Pouncy in 1855.
One might also add that as dichromated,exposed and washed Gloy works better as a
base than hardened gelatine in making gum prints,
it also works just as well as a final glaze to achieve your objective.
Terry King