GStreamer Isn't the Same Thing as PulseAudio
Whereas PulseAudio is a sound server, GStreamer is a multimedia framework.
PulseAudio runs all the time if it is set up to be used in your preferred environment. It intercepts all requests to output to the audio framework (usually ALSA) and decides what to do with them. In theory a sound server allows greater flexibility for mixing multiple audio streams (the old OSS framework would not mix multiple sources at all without a sound server). (I figure you know this, but the explanation may help someone else who's reading.)
Gstreamer, on the other hand, doesn't run at all unless you use a program that depends on it, or is set to output to it. Gstreamer basically can function as the engine for any number of multimedia programs. That is, it is software that converts files or data streams to sound and/or video. Competition for Gstreamer would be programs like Xine, MPlayer, and VLC. For instance, Totem can be run using either the Xine engine or the GStreamer engine. The difference between GStreamer and other multimedia engines is that, while competing engines generally have an associated piece of user interface software (like Xine-ui, GMPlayer, or the regular VLC program; mplayer has a built in command line interface as well), GStreamer only provides the engine.
