Auto-populate a field with an attribute from an IdP

Hi all,
I have a form that’s behind authentication to a Shibboleth IdP - every visit to the form has a login session behind it, and that session contains a user ID attribute (“uid”) that I could presumably pull into my form. I’m not quite sure how to do that, however.
I’ve confirmed with our IdP admins that the “uid” attribute is released and available to me, and I’ve read the GF documentation that seems relevant – merge tags appear to be at least part of the answer. However, those appear to be ready out of the box for logged-in users who came in via Wordpress authentication, which isn’t relevant to me.
What’s the missing piece(s) of the puzzle here – how/where do I connect the dots and map my attribute from the IdP to a field in Gravity Forms?
Thanks for any guidance.