The following uses the fancyBox jQuery plugin:
http://fancyapps.com/fancybox/. Make sure to download the entire package with css.
This method is from Design from Within's Flexibility package (available via package management for fresh MODX installs only). Get the source code in full without installing Flexibility here:
https://github.com/DESIGNfromWITHIN/Flexibility
<link rel="stylesheet" href="[[++site_url]]assets/templates/yourTemplate/stylesheets/jquery.fancybox.css">
<script src="[[++site_url]]assets/templates/yourTemplate/javascripts/jquery.fancybox.js"></script>
Call the gallery in your template with this chunk:
$pageGallery :
[[!GalleryAlbums? &toPlaceholder=`GalleryAlbums` &limit=`0` &albumCoverSort=`rank` &prominentOnly=`0` &rowTpl=`newGalAlbumRowTpl` &thumbWidth=`195` &thumbHeight=`140`]]
[[!Gallery? &checkForRequestTagVar=`1` &toPlaceholder=`Gallery` &useCss=`0` &containerTpl=`newGalleryAlbumTpl` &thumbWidth=`195` &thumbHeight=`140` &thumbTpl=`newGalItemThumb` &imageWidth=`800` &imageHeight=`800`]]
[[!If? &subject=`[[+Gallery]]` &operator=`isempty` &then=`[[+GalleryAlbums]]` &else=`[[+Gallery]]`]]
create new chunks (replace new with your own prefix and change the above chunk to match)
$newGalAlbumRowTpl :
<div class="albumPreview">
<h3><a href="[[~[[*id]]?&[[+albumRequestVar]]=`[[+id]]`]]">[[+name]]</a></h3>
<div class="thumb">
<a href="[[~[[*id]]?&[[+albumRequestVar]]=`[[+id]]`]]"><img src="[[+image]]" alt="[[+name]]"/></a>
</div>
<div class="desc">
<p>[[+description]]</p>
<a href="[[~[[*id]]?&[[+albumRequestVar]]=`[[+id]]`]]" class="view">View gallery</a>
</div>
</div>
$newGalleryAlbumTpl :
<h3>[[+album_name]]</h3>
<p>[[+album_description]]</p>
[[+thumbnails]]
$newGalItemThumb :
<div class="[[+cls]] gallery_item">
<a href="[[+image]]" rel="fancybox" title="[[+description]]">
<img class="[[+imgCls]]" src="[[+thumbnail]]" alt="[[+name]]" />
</a>
[[+name]]
</div>