You need to create a PAM entry for gradm similar to any other service that uses PAM (see viewtopic.php?f=1&t=2831). We don't provide one by default as each user will likely want some different authentication method. You also need to ensure the "P" flag is on the special role for which you want to use PAM authentication. FWIW, I don't recommend this for a fully-privileged admin role.
Once you have it working, feel free to update the wiki.