You have it
The GMPlayer package is definitivly available and much likely already installed if you are using a GNU/Linux distribution.
 Plays everything
The Movie Player will play any video file given to it even if they are partially broken. GMPlayer is just a fancy GUI for this player, which means that GMPlayer also plays everything from XViD to DVD to MP4 files.
There are many skins available for GMPlayer. These can make GMPlayer look very fancy.
However, the file picker does not use QT or GTK or anything other cool filebox code. It looks unimpressive and remains hard to use regardless of very fancy skin choosen.
 In bullet summary
- GMPlayer is nice if you feel like playing a video file
- The playlist support & filemanager is redicilous. If you want to have a easy-to-use playlist with a mix of audio and video files then VLC is much likely a better choice. GMplayer does not, for all practical purposes, have a playlist.