Select a disk, directory or file in the GTKtalog's main window for which you want to set a category and/or a description.
Right click to get the contextual menu and choose "Edit disk/directory/VFS/file".
If you check "Edit description", type the description in or select an existing one (right down arrow).
If you check "Edit category", select a category (right down arrow). Click on the "..." button to open the categories window. See Section 7.1. Save the change(s) clicking on the "OK" button.