See https://wiki.debian.org/ListInstalledPackages You can run in the terminal dpkg -l > packages.txtto write information about the installed .deb packages to file packages.txt. Also run
dpkg-query -f '${binary:Package}\n' -W > packages.txt
to just write the names.
I tried to get it to work in Synaptic but it seems easier to use the
terminal.
