Start with checking out portage as a git submodule

$ git submodule update --init

Then update your ./portdir with

$ ./update-portdir.sh

At this point, you can already run the following script to get the list of
Debian source packages that can be mapped to Gentoo packages and also have
build dependencies that can be dropped in Gentoo:

$ ./find_reduced.py

It will print:
  - a list of Debian source packages with dependencies that can be dropped
  - a list of Debian packages that cannot be mapped to Gentoo packages
  - a list of Gentoo packages that cannot be mapped to Debian packages

Overview:

portage               - portage sourcecode
                          * retrieved as a git submodule
portdir               - database of ebuild files
                          * retrieved by ./update-portdir.sh
deb_source_pkgs.list  - list of interesting Debian source packages
deb2gen_mapping.list  - mapping of Debian source packages to Gentoo packages