Premise: When specifying the url type in Google Gadget, the contents of the remote server are displayed as is in the iframe. Therefore, if you try to obtain friend information using Ajax, it is necessary to cross domains, and communication between servers will occur via a proxy, making it useless without a RESTful API.

Apparently Orkut doesn't allow Content Type="url".

MYSQL database connection using PHP for my gadget? – Orkut Developer Forum | Google Groups

It seems that specifying Content Type="url" returns a 404. The fact that a 404 is returned is a bug, but once Content Type="url" starts working, it can be confirmed that accessing the RESTful API via a proxy is required to use OpenSocial across domains. It is unclear whether this is a temporary measure until the RESTful API is officially released, but the premise appears to be correct.