kdiff3-qt does not provide desktop icon
Binary package hint: kdiff3
kdiff3-qt does not provide a desktop file. The desktop file provided in the kde variant has some special parameters in the Exec= part which seem to confuse gnome, at least it does not start while this parameters are there.
And last, Categories are missing completely inside the desktop file.
I derived a new desktop file from the provided one which works on gnome and puts kdiff3 under category development.