First, copy the link to the raw git-drush.sh file.
# Download the file
$ wget [link-to-raw-git-drush.sh-file]
# Move it to a directory where you store binaries, such as ~/bin or /usr/local/bin:
$ mv git-drush.sh /usr/local/bin/git-drush
# Make sure it is executable.
$ chmod +x /your/bin/dir/git-drush
# Run help to make sure it works and see how to use it!
$ git drush help