Adding option to Gallery shortcode

This code should work in your functions.php

add_shortcode('gallery', 'custom_gallery_function');
function custom_gallery_function($atts) {
    $user = wp_get_current_user();

    // if current user isn't admin, add posts to be hidden to exclude
    if(!in_array('administrator', $user->roles))
        $atts['exclude'] = $atts['exclude'] . ',' . $atts['hide'];

    // call the wordpress shortcode function
    return gallery_shortcode($atts);
}