gnash gstreamer support is relying on slot 0.10 (dead and deprecated for ages) and also gst-plugins-ffmpeg, that is broken with ffmpeg-3. Hence, I would simply disable that optional support Thanks
[master d716d7a] www-plugins/gnash: Disable optional gconf and gstreamer:0.10 support (#594884), prevent sandbox violations (#591014), handle gui options properly (#491120), use new ways for handling ffmpeg and openssl providers. 1 file changed, 288 insertions(+) create mode 100644 www-plugins/gnash/gnash-0.8.10_p20160329-r1.ebuild