I have geckodriver installed in my development directory
ls | grep geckodriver
geckodriver
geckodriver-v0.11.1-linux64.tar.gz
I also exported
You have to make sure that geckodriver is in your PATH.
In my case you can see that /usr/local/bin is already in my path:
$echo $PATH
/usr/local/bin:/usr/bin:/bin:/usr/sbin:/sbin:/usr/local/git/bin:/usr/local/mysql/bin
So simply moving geckodriver into /usr/local/bin makes it accessible.
mv geckodriver /usr/local/bin/