Our OpenSocial container implementation currently does not have all Google Gadgets methods (_IG_* which is not officially part of the API but we should have anyways since most people use and rely on some of that functions). This is something that should be fixed in the following versions, for now, what you can do is to include the javascript to fetch xml as part of your gadget as a workaround. I know it is not ideal and this feature request is already on our list of tasks for the OpenSocial support.