This is a self-reminder to check we support H.265 content with GStreamer properly after 1.4.x bumps if upstream supports it; or patch that support in for 1.4 series
My best guess is that it just needs a newer libav/ffmpeg than the current stable/testing ebuilds.
gst-plugins-bad 1.6 provides two plugins for this, de264 and x265.
*** Bug 528686 has been marked as a duplicate of this bug. ***
I just added gst-plugins-libde265 (for decoding) and gst-plugins-x265 (for encoding). I successfully read a video from libde265.org so I am closing this issue as fixed.