The source is accessible on github.
Provided you have git installed, you can get it with:
git clone --recursive https://github.com/Gwion/Gwion
then change to the source directory
At this point, you may want to configure the build. In this case, have a look at the configuration page.
You may need root privilege to do this.
This is not strictly necessary, as you can run the program by typing
gwiondirectory. It is a good idea to install it, however, so that it can be run in any folder on the system.