tl;dr Start with fresh Raspbian and do not alter
/etc/apt/sources.list to install GStreamer. Trying to fix APT repository is possible, but usually time-consuming task.
You can install GStreamer from the official repositories (as suggested in this answer):
sudo apt-get install gstreamer1.0-tools
Regarding your question: you need to add space between
man sources.list shows the following structure for the
deb [ options ] uri suite [component1] [component2] [...]
APT tries constructs an exact URL to the packages using:
In case of GStreamer which is served from
http://vontaene.de/raspbian-updates/dists/main/, the correct setting for
deb http://vontaene.de/raspbian-updates/ . main
It translates to http://vontaene.de/raspbian-updates/dists/./main/ - a reachable addres.
After setting the repository, you'd need to configure the keys. Anyway, currently setting this is unnecessary, see the top of the answer.