Your example:
http://www.atkgallery.com/gctgp/002/31/NC3BajJxkTq/
my guess is a complicated algorithm generated using the
002 and 31. For example galleries containing 001 only contain 8 digits
rather than 11. So I don't think anyone will crack the code...
However, if all your looking to do is browse through random ATK galleries use Google.
Try the following search inquirie:
site:
http://www.atkgallery.com
If you have firefox and the thumbs extension, enjoy ; )