You don't have to download the gcc source code from the gcc site. You can install the compiler using the apt-get command. Run the following commands to install the necessary files to compile programs:

sudo apt-get install build-essential

This should install the gcc along with g++ on your machine.

You could take a look at mplayer or more specifically mencoder which comes with it to do your movie file conversion. Just be aware that it would be better to compile and install using source code provided from the mplayer site instead of using the default mplayer that comes with ubuntu. You will also need to get some codecs from the mplayer site to open rmvb filers. There's no gui for the encoding part so you will be using the terminal to enter the commands to do the conversion.