Errors after stable 2020-08-28 Update | in gdm.service which seem related to PAM changes

Please double-check and use the files from revision 6 as you still have some mixture, which is not working!

etc/pam.d/gdm-autologin
etc/pam.d/gdm-fingerprint
etc/pam.d/gdm-launch-environment
etc/pam.d/gdm-password
etc/pam.d/gdm-smartcard

sudo pacman -U http://mirror.easyname.at/manjaro/pool/sync/gdm-3.36.3-6-x86_64.pkg.tar.zst

As some is totally off on your end.