Is there a good, simple commandline player that uses gstreamer?
gst123 is command line music player that uses gstreamer. I have not messed with it, I generally use MOC.
This wasn't available back in 2010, but since GStreamer 1.2 ca. 2013, there is a utility called gst-play. It is a simple utility that can play an individual audio file, or recurse through directories and play all the audio files it finds. E.g.
gst-play-1.0 /path/to/media.mp3. See
man gst-play-1.0 for more information. Also see
gst-play-1.0 --help for other options that aren't in the man page (including playlist from file). It's officially intended for "testing purposes".