net-wireless/gnuradio and a number of related ebuilds are python2.7 only. Upstreams are currently working or have already published python3 compatible code. Due to dev-python/cheetah vs. dev-python/cheetah3 blocker, gnuradio and all depending packages need updating at the same time, and will be p.masked until the migration is complete. This is the current state of things regarding python3 compatibility, will update with new information as it comes up: net-wireless/gnuradio: since 3.8.0.0 net-analyzer/gr-fosphor: in git master net-wireless/gr-air-modes: in git master net-wireless/gr-baz: https://github.com/balint256/gr-baz/issues/61 net-wireless/gr-doa: unknown net-wireless/gr-foo: unknown net-wireless/gr-ieee802154: in git master net-wireless/gr-iio: unknown net-wireless/gr-iqbal: since 0.38.0 net-wireless/gr-osmosdr: in gr3.8 branch net-wireless/gr-paint: unknown net-wireless/gr-rds: in git master net-wireless/gr-rftap: unknown net-wireless/gr-specest: in git master net-wireless/multimode: unknown, dead upstream
net-wireless/gr-osmosdr update: version 0.2.0, released on 2020-02-16, is the first (and so far the only) one to require python3 and gnuradio-3.8.
Old gnuradio removed in commit f87b58e64e4da6792adc0ef4cb9827876205513c.