On an Ubuntu system everything you need should be installed. If not, try this:
sudo apt-get install libgstreamer0.10-0 libgnomeui-0
Similarly, on Fedora everything should already be installed, but if not, try this:
su -c yum install libgnomeui gstreamer
The GSSMP developers recommend storing your music in Ogg Vorbis format. You almost certainly already have everything you need installed for this (once you've got gstreamer) but if not, on Ubuntu try:
sudo apt-get install gstreamer0.10-plugins-base
On Fedora, try this:
su -c yum install gstreamer-plugins-base
Please do send in instructions on installing these prerequisites for other distributions - we'd love to add them here.