Right now, moving image files around on the file system will break Catalogs. 
This is true whether the movement is done from within gThumb or by other means
(command line, other file managers, etc.).  

While I don't expect that gThumb could magically update catalogs while other
programs change the contents of the filesystem underneath it, it should be smart
enough to incorporate it's knowlege of file movements that _it_ performs and the
state of image catalogs.  

When one or more files is moved by using gThumb (via Edit -> Move... or by a
context menu), the appliation should compare the (list of) pathname(s) that it
is moving to it's internal lists of the files that make up Catalogs.  If it
finds that there are any matches, it should update the Catalogs with the files'
new paths.