For now I created a Script which takes care of grommunio-index and also the missing NGINX-Modules.
https://github.com/crpb/grommunio/blob/main/debian/alien8.sh
I also created a new thread but still waiting on a reply from the Grommunio-Team.
I even opened a ticket which refers to that thread if someone could make an official statement but haven't heard anything ... "Ticket#81508 - created Sunday 25. Sep 22:06"
FYI: You will need to updated the package yourself for now.