I'm familiar with Gaia, yeah. Basically I have a Widget application and I want users to be able to give other google identities access to Widgets. Typically, the users would grant access to other users (or service accounts, or google groups) via the email address. On my application, I can't grab the Gaia ID from the email address so the Subject Reference has to be the email address itself, which isn't recommended (https://developers.google.com/identity/sign-in/web/people)