dump(gallery_images)
array (size=2)
  'img' =>
    array (size=15)
      0 => 'path-to-file.jpeg'
      1 => 'path-to-file.jpeg'
      .....
   'alt' =>
    array (size=15)
      0 => 'img 1 alt text'
      1 => 'img 2 alt text'

{% for gallery_image in gallery_images %}
  <li>
      <img src="{{ gallery_image.img }}" alt="{{ gallery_image.alt }}" />
  </li>
{% endfor %}

{{ gallery_image.0 }}
{{ gallery_image.1 }}