I went through all this several times while trying to narrow down the repro steps, and it's all very bizarre. Somehow I managed to get gdm working again, and then in a logged in gdm session, I enabled fingerprint, and exited from pam-auth-update, then ran pam-auth-update again and disabled fingerprint and re-enabled register user sessions which had once again somehow magically gotten disabled, but then I ran pam-auth-update a third time and register user sessions was STILL disabled even though I'd just enabled it, and then when I rebooted GDM wouldn't work again. So at that point I logged in on a VT and ran pam-auth-update again and selected register user sessions again, and then just out of curiosity ran pam-auth-update again and this time register user sessions was still selected, so I exited and rebooted and GDM worked again.
Actually, it's even weirder than that.
I went through all this several times while trying to narrow down the repro steps, and it's all very bizarre. Somehow I managed to get gdm working again, and then in a logged in gdm session, I enabled fingerprint, and exited from pam-auth-update, then ran pam-auth-update again and disabled fingerprint and re-enabled register user sessions which had once again somehow magically gotten disabled, but then I ran pam-auth-update a third time and register user sessions was STILL disabled even though I'd just enabled it, and then when I rebooted GDM wouldn't work again. So at that point I logged in on a VT and ran pam-auth-update again and selected register user sessions again, and then just out of curiosity ran pam-auth-update again and this time register user sessions was still selected, so I exited and rebooted and GDM worked again.
There is something very bizarre going on here.