> Is this something that's not possible? What would be required to itemize 
> only the new packages separate from all others?

I am not going to get into the business with codifying your specific use cases into gslapt.  

I'm not sure if I have your definition correct, buf if you want a list of packages that are
available but do not have any version of it installed on the system, then you can easily get
that list from the command line with something like this:

for pkg in $(slapt-get --available | awk '{print $1}' | sort | uniq | sed -re
"s/(.*{1,})\\-(.*[\\-].*[\\-].*)/\1/");do if [ ! -f /var/log/packages/$pkg\-* ]; then echo
$pkg;fi;done