Description

The GStreamer library may be needed by some games and multimedia applications (including PowerPoint) to play audio and/or video content.

GStreamer is a Unix multimedia framework which CrossOver uses to allow Windows applications to encode or decode videos and audio files and streams. However to do so it needs plugins for each video and audio format. Those are provided by separate packages which you will also want to install. See the following related issues:
Missing GStreamer 1.0 Bad plugins
Missing GStreamer 1.0 Base plugins
Missing GStreamer 1.0 Good plugins
Missing GStreamer 1.0 Libav plugins
Missing GStreamer 1.0 Ugly plugins
Missing GStreamer 1.0 Ugly plugins (mpg123audiodec)

Resolution

Installing this library without also providing plugins to handle audio and video media is quite pointless. So use the plugin-specific pages above to fix this issue.