Using catch_that_image() for galleries?

The gallery is a shortcode, so you will need to parse the shortcode to grab the image referenced therein:

function pull_image_from_gallery(){
  global $post;
  $pattern = get_shortcode_regex();
  preg_match_all( "https://wordpress.stackexchange.com/". $pattern .'/s', $post->post_content, $matches );
  foreach ($matches[2] as $k=>$v) {
    if ('gallery' == $v) {
      $atts = shortcode_parse_atts($matches[3][$k]);
      if (!empty($atts['ids'])) {
        $ids = explode(',',$atts['ids']);
        $first = $ids[0];
        $image = wp_get_attachment_image_src( $atts['ids'] );
        if (!empty($image)) {
          // do what you want with image
          return $image[0];
        }
      }
    }
  }
}

I think the code is fairly easy to follow but…

1. grab the regex that Core uses to extract shortcodes
2. If there is a shortcode, parse the attributes
3. Grab the first ID and get the image ( or try
   get_attachment_image_url() if more convenient )