Rails each loop insert tag every 6 items?

You can use Enumerable#each_slice in conjunction with #each to avoid inline calculations. each_slice breaks the array into chunks of n, in this case 6.

<% @images.each_slice(6) do |slice| -%>
  <div class="gallery">
    <% slice.each do |image| -%>
      <%= image_tag(image.url, :alt => image.alt) %>
    <% end -%>
  </div>
<% end -%>