Currently, a catalog is a list of images (pathnames).  If (via gThumb or
otherwise) I move one of the image files that makes up the catalog, gThumb will
just silently remove the image from the catalog.  

I presume that gThumb discovers that the file is not present by trying to
display the image and getting an "file not found" error.  At present, that error
is not made visible in the GUI.  

I suggest that gThumb provide warning to the user that the images have been
removed from the catalog by adding a notice to the Status bar at the bottom
right of the window.  Right now, it says (for example) "26 images (31.3 MB)." 
Before I removed a test image, it said "27 images (and I forget what the was)."  

Suggested format:  when gThumb detects that an image file which was in its
catalog listing is no longer present, it should say something like:

26 images (31.3 MB) found - 1 image removed

The "1 image removed" would be a link (button?) that, when clicked would present
to the user a list of the filenames that _were_ in the catalog but now have been
removed.  This would at least allow the user to manually search for and re-add
the image to the Catalog.

Right now the program is misleading.  It knows that something bad happened (an
image has gone missing and (perhaps) a presentation might now go badly) but it
is not providing any warning or feedback to the user.