Global File System (GFS) is a shared disk file system usually for linux clusters. If you are trying to backup these shared folders then you can install the ralus agent on the linux server and do the complete backup of the linux machine. I don't think you can backup these as cifs shares by adding in the user defined selection list.
If you are trying to target the backup to GFS then you have to check if you are able to create the B2D folder on GFS. Usually we are able to create it on NAS and SAN but not sure if we can on GFS