For our application we are using the Graph API to fetch all rooms in a tenant with application permissions.
Rooms that are marked as "Hidden from GAL" can currently not be retrieved via the Places API endpoint (GET /places/microsoft.graph.room).
Therefore it is currently not possible to get those "hidden" rooms via the Graph API in any way whatsoever.
It would be nice to have the Places API to also return "hidden" rooms along with a new property on the room resource type (e.g. "HiddenFromGAL") which would signalize that flag.