As opus is now bitstream-frozen and in the process of becoming an IETF standard, the appropriate gstreamer plugin should be added to the tree. It is part of gst-plugins-bad-0.10.23, which is not yet in the tree, see #418075
Opus is now an IETF standard. What complications are involved with bumping a gst-plugins-bad ebuild?
media-plugins/gst-plugins-opus-0.10.23 and 1.0.3 are now in portage. Sorry for the wait!