This didn't actually work for me - not sure if anyone else is having the same issue.
I tweaked the second row from gsap-js, to gsap-script and it started working straight away!
wp_enqueue_script( 'gsap-js', 'http://cdnjs.cloudflare.com/ajax/libs/gsap/2.1.1/TweenMax.min.js', array(), false, true );
wp_enqueue_script( 'gsap-script', get_stylesheet_directory_uri() . '/js/gsap-scripts.js', array(), false, true );