i have in the preferences -> convertesrs -> gnumeric
just the entry
ssconvert --export-type=Gnumeric_html:latex $$i $$o
and i have ssconvert somewhere in directory that is on my search path (i.e. if i enter at a terminal prompt
i get a reasonable reply.
for me, it works (but the spreadsheet is not shown in the preview, it is just included in the pdf).