GStreamer plugins for FFmpeg Formula was removed
Last version was
gst-ffmpeg@0.10 requires the following formulae to be installed:
pkg-config 0.29.2 Manage compile and link flags for libraries
gettext GNU internationalization (i18n) and localization (l10n) library

Recent formula history

Mike McQuaid Remove all versioned unversioned formulae. (#11618)
ilovezfs gst-ffmpeg@0.10: add description
ilovezfs Formula/gst-ffmpeg@0.10: make not keg_only
ilovezfs gst-ffmpeg@0.10: use https for homepage
ilovezfs gst-ffmpeg@0.10: add a test

Formula code at GitHub