Interesting. One my system gdm has a shell of /bin/false, so an su to that account silently fails. I could change the shell for testing purposes and try it that way. If I remove the existing gdm db and try to accomplish this with sudo, I get a permission denied message: linux-jzs7:~ # sudo -u gdm /usr/bin/dconf update warning: Failed to create file '/etc/dconf/db/gdm.EDVONX': Permission denied