an ebuild for gst-plugins-jack is needed: Nothing fancy. it need to DEPEND on media-sound/jack-audio-connection-kit. Tested on x86. Also, this should made into a useflag to gstreamer-plugins. If no one minds I'll commit this myself.
the jack plugin in gstreamer is heavily broken and bitritten and will not be fixed until the 0.9 series therefore its worthless to have in portage, you may have more mileage using the alsa-lib plugin for jack and using gstreamer's alsasink
*** Bug 97788 has been marked as a duplicate of this bug. ***