Kubuntu ships with a modified version of KDE’s plasma that has Google Gadget support disabled. If you care, their justification for it is here. I have wanted to add Google Gadgets seemlessly to my desktop for some time and had hoped that the new version 9.04, Jaunty Jackalope would fulfill my dreams. As of Release Candidate 1, however, it appears it will not.
Nevertheless, hope is not lost. Someone has put together a nice quick and easy guide to getting Google Gadgets into Jaunty’s plasma desktop.