From Gnash Project Wiki
Gstreamer is a multimedia framework which Gnash can use for decoding audio and video.
Gstreamer itself cannot decode anything, so it needs some appropriate decoding-plugins to do the work for it. Remember to install them if you use Gnash with Gstreamer enabled. To get the best out of Gnash's gstreamer-parts, it is recommended to install the gst-plugins-good, gst-plugins-good and gst-ffmpeg plugins packages.
- The Gstreamer-sound handler uses the available plugins to decode the audio, so it might not work if some important plugins are missing.
- The Gstreamer-sound handler is known to have problems when rapidly starting playback of multiple sounds, and has a certain initializing delay.