If Gtk-Gnutella restarts a download this means the file did not match the advertized SHA1 checksum. This does not necessarily mean that the file unusable but the file is somehow corrupt. Such files are moved to the ~/gtk-gnutella-downloads/corrupt/ directory unless you configured Gtk-Gnutella to delete such files. You can get cancel any further downloads of the file by clicking on "Purge selected" in the Downloads->Files display.