I have a Google Gadget with Content type=URL that suddenly stopped working. It was running in a Google Site (as the Gadget container). I can access the URL easily outside the scope of the gadget container. Did Google Sites Gadget Container change support for content type URL gadgets?
In the latest version of browsers
non-secure contents are not allowed. Please check your gadget URL and also for any other locations in the gadget where URLs are specified with HTTP. Changing those to HTTPS or to protocol independent would fix your issue.