I was looking for a simple solution to add a lightbox to the standard Ghost gallery, I couldn't find one, so I put together the code below.

The following bit of jQuery and CSS when added to the Code Injection, Post Footer will add a lightbox style function to the post gallery.

I've tested this with the following free themes:

Casper Version 2.9.7
Massively Version 1.0.1

If you have tested it with a premium field and would like me to note it here, please let me know via the comments box below.

Here's a (hopefully) working example:  


<script>
  $(function() {
      $( ".kg-gallery-container" ).prepend( "<div id='kg-gallery-zoom' style='display:none;'><img src=''/><div id='kg-gallery-buttons'></div></div>" );
      $( ".kg-gallery-container .kg-gallery-image img" ).each(function(e){
        var thissrc = $(this).attr('src');
        var label = e + 1;
        $( "#kg-gallery-buttons").append('<span class="kg-gallery-btn" data-src="'+thissrc+'">' +label  +'</span>');
      });
      $( ".kg-gallery-btn").click(function(e){
          e.preventDefault();
          $( "#kg-gallery-zoom img").attr('src',$(this).data( "src" ));
      });
      $( ".kg-gallery-image img").click(function(image){
          $( "#kg-gallery-zoom img").attr('src',$(this).attr('src'));
          $( "#kg-gallery-zoom").fadeIn();
          $(".floating-header").fadeOut();
      });
      $( "#kg-gallery-zoom img").click(function(image){
          $( "#kg-gallery-zoom img").attr('src','');
          $( "#kg-gallery-zoom").fadeOut();
          $(".floating-header").fadeIn();
      });
  });
  </script>

  <style>
  #kg-gallery-zoom {
      background-color: rgba(0, 0, 0, 0.8);
      height: 100%;
      left:0px;
      overflow: hidden;
      padding:30px;
      position: fixed;
      top:0px;
      width: 100%;
  }
  #kg-gallery-zoom img {
      cursor: -moz-zoom-out; 
      cursor: -webkit-zoom-out; 
      cursor: zoom-out;  
      height: auto;
      height:auto;
      left: 50%;
      max-height:80%;
      max-width:80%;
      position: absolute;
      top: 0;
      top: 5%;
      transform: translateX(-50%);
      width:auto;
  }
  .kg-gallery-image img {
      cursor: -moz-zoom-in; 
      cursor: -webkit-zoom-in; 
      cursor: zoom-in;  
  }
  .kg-gallery-btn{
      background-color: #000;
      border-radius: 50%;
      color: #fff;
      display: inline-block;
      height: 30px;
      line-height: 30px;
      margin: 0.25em;
      text-align: center;
      width: 30px;
  }
  .kg-gallery-btn:active,
  .kg-gallery-btn:focus {
      background-color:#ccff00;
  }
  #kg-gallery-buttons {
      bottom:30px;
      margin-left:auto;
      margin-right:auto;
      position: fixed;
      text-align: center;
      width: 100%;
      z-index:99999;
  }
  </style>   

You can add the following code to the Code Injection Post Footer field on an individual post or the site wide Code Injection Footer field.

Latest at: https://github.com/stewartculshaw/GhostSimpleLightbox