I have made it!
I used .js from GP http://generatepress.com/wp-content/plugins/gp-premium/addons/generate-sections/inc/js/parallax.js?ver=1.3.04
Added it to my child theme using this code:
/*
* script for parallax
*/
add_action( 'wp_enqueue_scripts', 'my_parallax_method' );
function my_parallax_method() {
wp_enqueue_script(
'parallax-script',
get_stylesheet_directory_uri() . '/js/parallax.js',
array( 'jquery' )
);
}
After that, it was just a matter of selecting the right div classes and some css.
And now it moves and I’m happy… π