Whole document tree
Where is this #%@$! catalog file
Each catalog file is stored somewhere on your hard drive. You are able to choose location when you save a new catalog file for the first time.
If you can't remember the location, take a look at File->"Recent files". It is probably there.
If it is, open it and look at GTKtalog's title bar once the catalog is loaded. It displays the full path to the catalog file.
If it's not, try to remember if you really saved it with GTKtalog exiting properly. To search throuh your hard drive take a look at the "find" man page.