Appstream metadata needs valid license
The Appstream metadata file for gparted is not used in Ubuntu because it is missing a valid metadata license.
http://appstream.ubuntu.com/cosmic/main/issues/gparted.html
https://www.freedesktop.org/software/appstream/docs/chap-Metadata.html#tag-metadata_license
By far, the most popular license used for this purpose is CC0-1.0