For what it's worth, it seems to work with the rear camera add-on. While I haven't used it to actually scan a QR code in Google Authenticator (because I went the manual route, before the camera had arrived), I can confirm that scanning QR codes with the rear camera does work elsewhere. Before installing the rear camera, trying to scan a QR code in Google Authenticator would just give me a black image. Now I get the live feed from the rear camera instead, which makes me believe it should work.
It would be nice though, if Google could enhance Authenticator with front camera support, not only for typical Gemini owners, but I do presume some tablets also only has a front facing camera, and some people might have a broken main camera...