If you run
# equery list gst-plugins-
you will get a list of all gst-plugins, i.e.
[ Searching for package 'gst-plugins-' in all categories among: ]
* installed packages
[I--] [ ~] media-libs/gst-plugins-base-0.10.8 (0.10)
[I--] [ ~] media-libs/gst-plugins-good-0.10.3 (0.10)
[I--] [ ~] media-libs/gst-plugins-ugly-0.10.3 (0.10)
[I--] [ ~] media-plugins/gst-plugins-a52dec-0.8.12 (0.

[I--] [ ~] media-plugins/gst-plugins-a52dec-0.10.3 (0.10)
[I--] [ ~] media-plugins/gst-plugins-alsa-0.8.12 (0.

[I--] [ ~] media-plugins/gst-plugins-alsa-0.10.8 (0.10)
[I--] [ ~] media-plugins/gst-plugins-cdparanoia-0.8.12 (0.

[I--] [ ~] media-plugins/gst-plugins-cdparanoia-0.10.8 (0.10)
[I--] [ ~] media-plugins/gst-plugins-dvdnav-0.8.12 (0.

[I--] [ ~] media-plugins/gst-plugins-dvdread-0.8.12 (0.

[I--] [ ~] media-plugins/gst-plugins-esd-0.8.12 (0.

[I--] [ ~] media-plugins/gst-plugins-esd-0.10.3 (0.10)
[I--] [ ~] media-plugins/gst-plugins-ffmpeg-0.10.1-r1 (0.10)
[I--] [ ~] media-plugins/gst-plugins-flac-0.8.12 (0.

[I--] [ ~] media-plugins/gst-plugins-flac-0.10.3 (0.10)
[I--] [ ~] media-plugins/gst-plugins-gconf-0.10.3 (0.10)
[I--] [ ~] media-plugins/gst-plugins-gnomevfs-0.8.12 (0.

[I--] [ ~] media-plugins/gst-plugins-gnomevfs-0.10.8 (0.10)
[I--] [ ~] media-plugins/gst-plugins-mad-0.8.12 (0.

[I--] [ ~] media-plugins/gst-plugins-mad-0.10.3 (0.10)
[I--] [ ~] media-plugins/gst-plugins-mpeg2dec-0.8.12 (0.

[I--] [ ~] media-plugins/gst-plugins-mpeg2dec-0.10.3 (0.10)
[I--] [ ~] media-plugins/gst-plugins-ogg-0.8.12 (0.

[I--] [ ~] media-plugins/gst-plugins-ogg-0.10.8 (0.10)
[I--] [ ~] media-plugins/gst-plugins-pango-0.8.12 (0.

[I--] [ ~] media-plugins/gst-plugins-pango-0.10.8 (0.10)
[I--] [ ~] media-plugins/gst-plugins-vorbis-0.8.12 (0.

[I--] [ ~] media-plugins/gst-plugins-vorbis-0.10.8 (0.10)
[I--] [ ~] media-plugins/gst-plugins-x-0.10.7 (0.10)
[I--] [ ~] media-plugins/gst-plugins-xvideo-0.8.12 (0.

[I--] [ ~] media-plugins/gst-plugins-xvideo-0.10.8 (0.10)
You will probably find some of these plugins are from version 0.8.11. These all need to be updated to version 0.8.12.
That should fix the problem.
Good Luck