layman -a sigrok
See http://wiki.gentoo.org/wiki/Layman for info on using layman.
Note: I don't do any cross-version testing (trying to use older versions of sigrok-cli with a newer libsigrok, for example). If you find a dependency on a specific version, please file a bug or make a pull request to add it!
For libsigrokdecode, it is necessary to select a PYTHON_SINGLE_TARGET of a python3
You can do this by
pyver=python_single_target_python3_3
usefile=/etc/portage/package.use
echo sci-electronics/libsigrokdecode $pyver >> $usefile
# also need to set for sigrok-cli and pulseview if they use
# libsigrokdecode
echo sci-electronics/sigrok-cli $pyver >> $usefile
echo sci-electronics/pulseview $pyver >> $usefile
Or something similar.