GRR is a room booking software.
GRR has a SSO configuration page in its administration panel.
Do not use Lemonldap mode, which is for a very old Lemonldap version, but HTTP authentication.
Set the default profile of connected users and which headers contains surname, firstname and mail.
GRR will check the username in REMOTE_USER, so use remote header conversion if you are in proxy mode.
Access rules:
Headers: