The gstreamer project maintains lists as part of the official documentation for the individual components and plugins. These lists consist not of file extensions but of codecs, containers and formats.
But you shouldn't need to have such a list in the first place, particularly not by file extension. If you encounter a file you can't play and there is a package that adds playback support for this format just install it. In a perfect world I'm pretty sure Ubuntu would ship with all codecs installed by default (or there would just be the one perfect format), because you as enduser should never bother with such an issue. Unfortunately there are patent issues, which is mostly why the restricted-extras packages and gstreamer-ugly exist. Also the code quality of some decoders does not meet the standards, the reason for packages like gstreamer-bad, but bad support might in some cases still be better than no support.
Your second question: Does gstreamer cover all my multimedia needs? Are there alternatives?
Gstreamer is the default multimedia framework used by Ubuntu and the applications in the default installation are chosen to integrate nicely into that environment. Gstreamer has become the de facto standard on the Linux desktop because of its functionality and it is well maintained. Xine was also a popular multimedia framework but it seems like it is no longer actively maintained. That is mostly it for frameworks which are comparable to DirectShow / Media Foundation or QuickTime.
On the lower level you might have noticed there is FFmpeg which is used as a component in gstreamer and itself provides a lot of functionality. VLC and MPlayer/MPlayer2 are two media players based upon FFmpeg and don't make use of gstreamer. However if the default media player covers all your needs, you don't have to install those.
But anime fans for example would enjoy Advanced Substation Alpha (ASS) subtitle format support and H.264 High 10 profile (aka 10-bit) video playback. For this use case the Motu Media launchpad group provides PPAs with daily MPlayer2 builds and the SMPlayer2 front end. However this is bleeding edge and can break sometimes.
Encrypted DVD playback is disabled by default, the community documentation has a page on that topic. You basically need to install a package AND run a script.
FFmpeg installs by default with some functionality stripped of, community documentation has a page on that topic, too. Those instructions are not up to date, but installing a few of the extras packages, with newer versions will pull the others back in as dependecnies.