I was finally able to make gsvideo work on OSX (tested on Leopard with an 20″ iMac)! The problem was not as much as with gsvideo itself, which needed just some small changes in order to set the directory where GStreamer libraries get copied. The difficult part was to have GStreamer actually installed on OSX, for which I used MacPorts. This involves first installing the Apple development tools (XCode) and then getting your hands a little bit dirty in the command line. The good news is that the version of GStreamer available on MacPorts has been recently updated to the latest released one, so almost every plugin and codec is working right now (except camera capture).
This is the new version of GSVideo (0.3.11) that fixes the path issues on OSX:
Hopefully, in a not too distant future, it will be possible to install GStreamer by means of a binary package. As for now, MacPorts seems to be the only way to do it. Here is the homepage of MacPorts, and here a short tutorial about it.
A full install of GStreamer involves installing the following packages: gstreamer, gst-plugins-base, gst-plugins-good, gst-plugins-bad, gst-plugins-ugly and gst-ffmpeg. And because of all the dependencies that these packages require, the install ends taking up to 360MB!