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
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

Above script depends on the following data files to be present:

portage               - portage sourcecode
                          * retrieved as a git submodule
portdir               - database of ebuild files
                          * retrieved by ./update-portdir.sh
reduced_gen_deps.list - Gentoo packages with reduced dependencies
                          * generated by ./gentoo_dep_list.py
gen_source_pkgs.list  - list of Gentoo source packages
                          * generated by ./gentoo_pkg_list_raw.sh
deb_source_pkgs.list  - list of Debian source packages
                          * supplied by yourself
deb2gen_mapping.list  - mapping of Debian to Gentoo packages
                          * created manually and/or with the help of
                            ./find_pkg_matches.py

deb_source_pkgs.list is filled by the user. All other *.list files can be
regenerated by the following scripts:

$ ./gentoo_dep_list.py

   It writes all source packages with reduced build dependencies plus the build
   dependencies that can be dropped to reduced_gen_deps.list. Execution takes
   about 2.5 hours.  

$ ./gentoo_pkg_list_raw.sh

   It writes to gen_source_pkgs.list a list of all available Gentoo packages.

$ ./find_pkg_matches.py

   Inspects the current content of deb2gen_mapping.list and prints a list of
   Debian packages that miss a mapping to Gentoo packages as well as a list of
   suggestions of Gentoo packages that sound similar.