Due to checks in the code probably introduced around bug 534362, gnuradio-3.7.7 checks for swig-3.0.3 or 3.0.4 and fails during src_compile (see https://gnuradio.org/redmine/projects/gnuradio/repository/revisions/0d219496168402467657c441b72444671b95580a/diff/). It looks like the swig dep needs to be updated to exclude the 3.0.4 version still in-tree (3.0.3 was removed circa bug 534362). For whatever reason portage-2.2.18 didn't update it prior to updating gnuradio.
addressed during the 3.7.7.1 update. thanks for the documentation in the bug!