GMPlayer (also known as "mplayer-gui") is a aging graphical interface for the not-so-updated MPlayer video player. It was something we considered a great choice around 2005-2007. It is no longer relevant and not worth consideration.
GMPlayer it's interface supports skins and there are a few to choose from. The default and most of those who are available were designed in an era where computer monitors used lower resolutions than modern monitors. Here is why that is a problem:
Look closely and you will see a control GUI in the lower right corner. It's barely larger than the mouse-pointer and it's size can not be changed or scaled.
GMPlayer was a great piece of software back in the day. Today it's no longer a viable alternative.