Gentoo: Please select pinentry-gnome3 as default pinentry provider

Create `/run/host/etc/portage/package.use/gcr` with the content of: app-crypt/gcr gtk
app-crypt/pinentry gtk

Then rebuild: `sudo emerge -avuDN @world`. Now `eselect pinentry list` should also list `pinentry-gnome3`. To select it:
sudo eselect pinentry set pinentry-gnome3